A:冬天清晨枯草上会结霜,说明空气中含水蒸气
B:木材能在空气中燃烧,说明空气中含氧气
C:正常的雨水呈酸性,是因为空气中含二氧化碳
D:利用空气成分可制取氨气,因为空气中含氢气
—That's quite a long time. There must be too many people waiting in line.
18里面去掉____还剩10。