单项选择题
The weight of liquefied petroleum gas vapors as compared to air is（）.
A.variable B.the same C.lighter D.heavier
单项选择题 The vapor pressure of a substance（）.
单项选择题 The vapor pressure of a gas is the pressure necessary to keep it in a(n)（）.
单项选择题 The tops of vents from gasoline tanks should terminate（）.
