— Who do you think ______ the book here?

— It may be John. He was sitting here all the morning.

A．was leaving

B．leaves

C．had left

D．has left

将受热零件的热量传递出去，保证柴油机正常工作的是（）。

A.润滑系统

B.进气系统

C.排气系统

D.冷却系统