阅读C522型立式车床控制原理图时要先看（）。
A．主电路
B．辅助电路
C．照明电路
D．整流电路
参考答案：A
Sailing down the Yellow River,______.
A. the boat hits a rock
B. an accident had happened
C. the boat hit a rock
D. a terrible accident happened
选择性培基养
