He works in a factory now, but he   on a farm for 20 years.

A．worked

B．has worked

C．had worked

D．had been working

（）指排放后阀瓣重新与阀座接触，即开启高度为零时的进口压力。

A、排放压力

B、回座压力

C、整定压力

D、背压力