一平面余弦波波源的振动周期T=0.5s，所激起的波的波长λ=10m，振幅为0.5m，当t=0时，波源处振动的位移恰为正向最大值，取波源处为原点并设波沿x轴正向传播，此波的波动方程为（）。

A.A

B.B

C.C

D.D

The color of rockets，shells，or rocket parachute flares used to indicate that the vessel is in distress and requires immediate assistance is（）.

A.white

B.green

C.red

D.yellow