问题
选择题
Nowadays, a large number of college students home and abroad ____ some part-time jobs in their spare time. [ ]
A. is doing
B. does
C. do
D. did
答案
答案：C
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Nowadays, a large number of college students home and abroad ____ some part-time jobs in their spare time. [ ]
A. is doing
B. does
C. do
D. did
答案：C
，要使两球在碰撞前的总动能为最小，试求碰撞前两小球的速度大小．