If we throw away too much waste and can’t keep good ecosystem,we _________ our future.

A．must have destroyed

B．could be destroying 

C．may be destroying

D．must destroy 

柴油机的气阀机构根据其结构特点可分为（）。

A.不带阀壳式

B.带阀壳式

C.A＋B

D.带旋阀器式