It's raining now. I have to go to school _____.
A. by the car
B. in car
C. in the car
答案:C
在波的传播方向上,有相距为3m的两质元,两者的相位差为,若波的周期为4s,则此波的波长和波速分别为()。
A.36m和6m/s
B.36m和9m/s
C.12m和6m/s
D.12m和9m/s
雷达接收机在雷达系统中起到什么样的作用?