延长预热器使用寿命的途经有哪些？
参考答案：
主要有四个方面：限制空气预热温度；要有保护措施；改进结构；严格执行操作规程。
You work hard _____________ school. [ ]
A. in
B. on
C. at
QQ, ________ you can chat with your friends, makes communication easy.
A．how
B．where
C．that
D．which
