问题
选择题
My mother ______ when someone knocked at the door. [ ]
A. cooked
B. is cooking
C. will cook
D. was cooking
答案
答案：D
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My mother ______ when someone knocked at the door. [ ]
A. cooked
B. is cooking
C. will cook
D. was cooking
答案：D
时刻的图象，已知质点M的振动方程为
（cm），此波中质点M在
s时恰好第3次到达y轴正方向最大位移处，该波的波速为 m/s． 