（）不是摩托车挂档困难的原因。
A.离合器分离不彻底
B.离合器打滑
C.变速换档时操作动作不协调
D.变速鼓导销槽过度磨损
参考答案：B
计算复数5i-（2+2i）=_______
The series of valves used to control the return flow in well control operations is called the（）.
A.valve assembly
B.standpipe manifold
C.accumulator manifold
D.choke manifold
