DF12机车空压机是（）驱动的。
A.步进电机
B.交流电机
C.直流电机
D.交直流两用
参考答案：C
Prices ______ all the time. It's no wonder that we've spent so much money recently.
A．rose
B．had risen
C．were rising
D．have been rising
组织细胞在绝对不应期时其兴奋性的变化（）
A.为零
B.小于正常
C.无限大
D.大于正常
E.正常
