问题 多选题
下列说法正确的是(  )

A.在照相机的镜头前涂有一层增透膜,其厚度应为入射光在真空中波长的;拍摄玻璃橱窗内的物品时,往往在镜头前加一个偏振片以增加透射光的强度

B.变化的电场周围一定产生变化的磁场,变化的磁场周围一定产生变化的电场

C.当波源静止不动且发声的频率不变,而观察者匀速向波源靠拢时,观察者接收到的频率在不断地变大

D.无线电波的接受需要调谐与检波过程

答案

CD

问答题
阅读理解

阅读理解

      Dave and Bob like playing very much. They often came up to the city from the country on Sundays to do some shopping, had a meal and then went to the theatre in the evening.    

      One evening it was twenty-five past eight and they were waiting for the play to begin. The theatre was  full. Everyone was laughing, talking and eating something. Soon the red curtain(幕) went slowly up, the       lights in the theatre went out(熄灭 ) and the play began. Then everyone was quiet. They watched, listened and thought. But it was a sad play. Dave and Bob didn't like it.      

      Last Sunday they saw a happy one and liked it. After the play Dave and Bob took the last train to        leave the city at ten past twelve.

1. Dave and Bob often went to         .    

A. the city from the village on Saturdays    

B. the country from the town on Sundays    

C. the city from the country on Sundays    

D. the theatre from the shop on Sundays

2. They went to the theatre in the evening        .    

A. before they did some shopping    

B. after they had a meal    

C. before they had a meal    

D. in the country

3. The theatre is full of          .  

A. talking              

B. laughing    

C. eating                

D. people

4. When the lights in the theatre went out,the people         .    

A. were quiet            

B. stood up    

C. began to talk          

D. went out of the theatre

5. They enjoyed going        .    

A. to the theatre to see a happy play    

B. to the theatre to see a sad play    

C. to the country to do some shopping    

D. to the country to have a meal