My father usually goes to work by car, but ________ he goes by bus.

A. sometime

B. sometimes

C  some time

D. some times

运行中水泵机组出水压力摆动过大并且是有规律摆动，可能是因为（）所造成。

A.电机扫膛

B.水泵抽空

C.泵内进气

D.盘根压的过紧