活塞式发动机飞机其极限时速为（）公里。
A.1000公里/小时
B.500公里/小时
C.450公里/小时
D.700公里/小时
参考答案：D
Forty minutes _____ for the children to finish all the work. [ ]
A. is quite a long time
B. are quite long
C. was a quite long time
D. were quite long time
用艾氏剂法测定煤中全疏时，应注意哪些问题?
