换气效率
参考答案：
新鲜空气置换原有空气的快慢与活塞通风下置换快慢的比值。
_____ shall we do with the rubbish in the bag? [ ]
A. When
B. When
C. What
D. How
---How long have you been studying in No.2 Middle School?
---Less than half a year.
---So it will be two years and a half _______ you graduate from this school.[ ]
A.when
B.after
C.before
D.while
