简述冷段和热段。
参考答案：
从维护角度上分，将发动机分为冷段和热段两部分。冷段：进气道，压气机，扩压器。热段：燃烧室，涡轮，喷管。
He received a letter the day before yesterday ____ him to go back to his home as soon as possible. [ ]
A. asking
B. to ask
C. asked
D. having asked
爬坡试验包含（）试验。
A.最大爬坡度
B.爬长坡
C.爬坡耐久度
D.爬坡速度
