Up to Tuesday's attack, the worst school shooting in the U. S. was in March 1998 in Jonesboro,______two boys, aged 11 and 13, shot and killed a teacher and four girls.

A．while

B．where

C．when

D．then

维护修理时，车辆应选择平坦地点停放，变带器放入（）。

A、１档

B、倒档

C、２档

D、空档