As the busiest woman in Norton, she made ________ her duty to look after all the other people’s affairs in that town.

A．this

B．that

C．one

D．it

液控单向阀在船舶液压机械中常用来（）。

A．控制回油背压

B．在一定条件下锁闭油路

C．改换油流方向

D．作卸荷阀用