光在____ 中是沿直线传播的.光的反射分为____ 和____ 两类.
You'll never ______ wrong if you follow his advice.
The sports meeting was put off _______ the heavy rain.
力F→=(-1,-2)作用于质点P,使P产生的位移为S→=(3,4),则力F→质点P做的功为____
一根绳子对折两次后,用尺量长10厘米,原来这根绳子长____厘米.