液压系统基本回路按作用分，最主要的有几种？
参考答案：
主要有三类：即压力控制回路、速度控制回路和方向控制回路。
When I talked with my grandma on the phone, she sounded weak, but by the time we ______ up , her voice had been full of life.
A．were hanging
B．had hung
C．hung
D．would hang
钴-60原子核内的中子数应为()
A．27
B．38
C．30
D．32
E．33
