配气机构的任务是根据工作过程的需要适时地开、闭()。
A、进气门
B、排气门
C、进、排气门
参考答案:C
The ______ in e-mail messages has affected almost every computer around the world and has caused the damage of up to US$1 billion in North America.
A.illness
B.virus
C.weakness
D.attachment
建筑物的耐火等级分为( )。
A.一级
B.三级
C.四级
D.五级