——Do you think Peter is free today?
——__________. He is really busy preparing(准备) for the coming test these days.
A：Of course
B：I have an idea
C：1' m afraid so
D：I don' t think so
质量m=0.02kg的子弹以v0=300m/s速度射入质量为M=2kg的静止在光滑的水平桌面的木块，子弹穿出木块的速度v=100m/s，求：
—________!
