Some children are coming out of the building. Some of them ____ into the park.
A. go
B. is going
C. are going
D. going
The door is open. Sam is going ____ it.
A. to shut
B. shut
C. to shutting
D. shuts
What ____ the children ____? They are jumping off the tree.
A. is, doing
B. are, doing
C. are, do
D. do, do
圆轴扭转，其上各点都处于纯剪切应力状态
A. 对
B. 错
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