止回阀从构造上可分为()。
A.截门式;
B.摆动型;
C.压力式;
D.流量式。
参考答案:A, B
I haven't seen my old friends for years, I 'm looking forword _____ them. [ ]
A. to meeting
B. meet
C. to meet
D. meeting
泵的流量,压力下降的原因。