28．She’ll never forget her stay there ________ she found her son who had gone missing two years before.

A．that

B．which

C．where

D．when

液压动力转向系统中，（）是在驾驶人的操纵下，控制转向动力缸输出动力大小，方向和增力快慢的控制阀

A.转向器

B.转向控制阀

C.转向油泵