为防止空气预热器金属温度太低而引起腐蚀和积灰，在点火初期应将送风机入口（），以提高送风入口温度。
参考答案：热风再循环挡板打开，投入暧风器
— ____ is she?
— She is in the library. [ ]
A. What
B. Where
C. How
It ____ 2 days _____ we know the result of the exam. [ ]
A. is, when
B. was, before
C. won't be, when
D. won't be, before
