单项选择题

21()

A. is developing 
B. has developed 
C. develops 
D. is being developed

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单项选择题
In arithmetic the rules of addition are basic, and all the other rules are built on this ()

A. basis      
B. bases    
C. base    
D. basises
单项选择题
She told them where she()her son.

A. has last seen
B. had last seen
C. last seen
D. was last seeing
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