（）作用是控制液体压力，调节流量、改变液体流向的液压元件。
A.液压控制阀
B.液压动力源
C.执行元件
参考答案：A
The story showed marriage between people of different races ,_____ was against the law at that time.
A．who
B．which
C．whoever
D．whichever
科学决策
