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The automatic generation of multiple-choice item (MI) has attracted amounts of attention. However, only a limited number of existing research address automatic MI generation for prepositions, and even fewer consider learners’ need in the generation process. In this paper, we propose an approach to generate preposition MIs suitable for non-native English learners of different language proficiency. First we select sentences with similar difficulty level to that of a given textbook as stems by using the sentence difficulty model we constructed. Then, we use the Word2vec model to retrieve a preposition list of distractor candidates where three of them are chosen as distractors. To validate the effectiveness of our approach, we produce four tests of preposition MIs at different difficulty levels and conduct a series of experiments regarding evaluations of stem difficulty, distractor plausibility and reliability. The experimental results show that our approach can generate preposition MIs targeting learners at different levels. The results of distractor plausibility and reliability also point to the validity of our approach.

We propose an efficient quantum memory scheme with controlled reversible inhomogeneous broadening in an optical cavity.When the absorption per cavity round trip exactly matches the transmission of the coupling mirror(impedance matching condition),we show that the input light field can be completely absorbed by an inhomogeneous broadening atomic ensemble.From calculation we obtain that either a forward or a backward retrieval process the quantum memory efficiency can reach unity even for a small optical depth of the atomic system.If there is no cavity,100% efficiency is obtained only for extremely large optical depth.A high efficiency for the quantum memory at a small optical depth is easily achieved in practice.Therefore our proposal offers promising possibilities for the practical realization of high quantum memory efficiency.

Flicker is the serious defect of TFT-LCD.In this paper the relations among photo-Ioff,Vcom,flicker and Vgs had been observed through a series of experiments,and the mechanism explanation is demonstrated.The change of Vgs brings ΔVp,results in the shift of Vcom and flicker,however,this kind of flicker can be avoided theoretically by the Vcom adjustment if the uniformity of Vcom has no problem.The pixel's voltage holding characteristic has difference between the positive and negative frames due to the Photo-Ioff,so the further shift of Vcom and flicker are happened.The flicker phenomena can be improved by decreasing the value of Photo-Ioff or increasing the Vgl.

The discrete particles and their existing space are supposed as a continuous medium,and its equivalent dielectric constant of the composite medium is developed.By using this constant and the attenuations induced by rainfall recommended by ITU,the dielectric constant is completely determined in GHz wave band and its validity is tested.The analytical expression of complex scattering field for a target in rainfall is presented.So the complex differential scattering in rainfall is simulated.The influences of rain,frequency and polarization etc on the scattering field are researched.Computations show that the influence induced by the rainfall on the differential scattering cross section is about 10-3 dB,and the effect produced by the vertical polarization wave is bigger than that resulted by the parallel polarization wave.The obtained result provides a reference in precision guide and target identification.

The urban thermal environment is an important element for the urban ecological environment,urban climate and urban disasters.This paper selected MOD11A2,the MODIS LST night data to study the thermal environment evolution in Beijing,Shanghai and Guangzhou,which are the three major cities of China in the past decade.Three methods have been applied in the paper,Landscape centroid evolution,Landscape pattern index and spatial autocorrelation.Three main conclusions have been drawn as follows.Firstly the thermal landscape distributions in the three cities have moved from the suburb to the downtown.And the evolution trend of the thermal landscape is changed from the low temperature region,sub-middle temperature region to middle temperature region,sub-high temperature region and high temperature region.Secondly,among these five types of thermal landscape,the middle temperature region is the most prevalent.The urban thermal landscape fragmentation was highest in Shanghai among the three cities,and sub-middle and high temperature region has the highest fragmentation.The urban thermal landscape dispersion was highest in Beijing,and the dispersion of low and high temperature region was higher than the other types of thermal landscapes.Thirdly,thermal environment spatial autocorrelation analysis showed that the high-high temperature zones were adjacent,low-low temperature areas were adjacent,which are the main types in the temperature spatial agglomeration.And for Beijing and Guangzhou city,the high-high temperature zone located in the south of the city,the low-low temperature region located in the north.While,the spatial autocorrelation distribution of LST in Shanghai is more complicated.The distribution areas of high-high temperature varied among the three cities in the past decade.In Beijing,the distribution area increased shortly after decreasing,and in Guangzhou,the distribution area continued to decline,which preliminary reflects the heat island effect problem aggravated in Beijing,while weakened in Shanghai and Guangzhou. Through comparisons and analysis,the paper has provided a reference for urban planning and urban living environment improvements,but there are still some inadequacies to be further studied.Firstly,this study only selected the January night LST data in the three cities.Because the time factors,such as season,daytime and nocturne,will affect the urban heat environment pattern,the comprehensiveness of the thermal environment pattern changes need to be improved.In addition,the paper only selected the data in the period of three years,the evolution regulation of the urban thermal environment pattern is not precise.Secondly,the landscape of urban heat environment were impacted by many factors,including the pattern of landuse,urban surface construction,weather conditions,terrain,anthropogenic heat emissions factors and so on.The analysis between the urban heat environment and impact factors will help reveal the mechanism of urban heat environment and which will be studied further.

Based on that the potential functions and medium parameter are irrelative and expanding the electric fields inside outside the anisotropic medium target into series,the nth-order expressions both of scattering field from an anisotropic medium target and its inner elelctric field are developed.The transformation of the dielectric constant tensor is presented.When a plane electromagnetic wave incident in arbitrary direction,a general relation between the unit vector in propagation and that in polarization is expressed.Taking the magnetized cold plasma as an example,the first order scattering is developed and the error induced by the second order field is evaluated.In the THz wave band and optical wave band,the obtained result is simulated.Simulation results show that the differential scattering change sensitively with operating frequency and the polarization etc.;the non-diagonal elements of the tensor have no influence on the scattering;when the wave length and the size of the target are given,the simulations are correct in THz wave band and validate in other wave band.

To overcome the problem that traditional Mean-Shift based tracking method is easy to fail because of the rigid tracking window with the changing of object scales,a new algorithm to estimate the scale of the tracking window is proposed.It divides the template image and the current frame image into several concentric cirques.After calculating the comparability measurement of corresponding cirques between the template and the current image,it ascertains the bandwidth parameter of current frame.Finally,Kalman filter is adopt to finish the update of the template scale and the stable object tracking is realized.Experiments prove that the proposed algorithm is able to renew the template accurately when the scale of the object changes,and the tracking reliability is improved compared to the traditional Mean-Shift based tracking method.

Out-of-plane and in-plane reorientations of mesogens were observed upon a linearly polarized light irradiation of azobenzene containing side-chain liquid crystalline polymer,which were characterized by the transmission various and polarized optical microscopy observation of the pumped film.Higher power density polarized light(20 mW/cm2) irradiation on the film induces that a maximal value of the photoinduced transmission is firstly obtained and then a decrease in the transmission value was observered.This suggests an out-of-plain orientation happened.Out-of-plane orientation decreases the stability of the oriented mesogens,which is confirmed by the lost of the anisotropy when the exposure light turn-off.The in-plane and out-of-plane reorientation depend on the laser fluence.The photoinduced anisotropy measurements results reveal that a stable orientation occurred by using an unsaturated irradiation upon a low power density laser beam.The unsaturated irradiation enables to shorten 95% of the incident time and increases the orientation order 1.3 times as compared to that of full orientation.A quick and high stable bit recording is achieved by unsaturated orientation upon 4 ms light irradiation.

Azobenzene containing side-chain liquid crystalline polymer was synthesized by hydrosilation reaction of poly(methylhydrosiloxane) with the azo monomer.The chemical structure and its liquid crystalline properties were investigated by FITR,DSC and POM with a hot stage.Photo-alignment treatment was performed on the polysiloxane film by using a linearly polarized light source with the wavelength of 473 nm.A static contact angle mea-surement was carried out and the surface free energy was also calculated from the contact angles of a drop of water on the polymer film.The water contact angles was decreased from 94.5° to 76.5° after the light irradiation.We conclude that the changes in the hydrophilic of LC polymer were influenced by the surface compositions upon the photoinduced reorientation of the azobenzene side groups perpendicularly towards the film surface,which was confirmed by conoscopic POM observation results.