下列说法正确的是（ ）
A．光总是沿直线传播
B．光由水中进入空气中时，传播方向可能不发生变化
C．太阳和月亮都是光源
D．光不能在真空中传播
A、光在同一种均匀介质中是沿直线传播的，故A错；
B、当光由水垂直入射进入空气中时，传播方向不发生变化，故B正确；
C、月亮不是光源，是靠反射太阳光照亮大地，故C错；
D、在真空中的传播速度最大，等于3×108m/s，故D错．
故选B．
选择方框内所给词的适当形式填空。
We just want to say thank you for having us before we caught the plane last week. It was a nice evening, and we enjoyed _小题1:_ Peter and Sarah. With your help we were able to get to the airport with plenty of time. We often tried to get an earlier flight (航班) than usual, but it wasn't possible. We had a _小题2:_ holiday in Spain (西班牙). We just loved driving through the countryside, and we often _小题3:_ to walk round a mountain village. We met _小题4:__ friends, Bill and Sue, and they invited us to have a meal with them. They wanted us to stay with them. But we couldn't, as we had already booked (预定) two rooms in a hotel. The weather was nice. There _小题5:_ so much beautiful sunshine all time we were there. Leaving Spain was very sad. It made me want to cry. Anyway, we are looking forward to hearing from you, and hope to see you soon. Let me know if you're ever in the area. You must call in. Yours, Ray |