Gas Fills All the Space Available to It

Gas fills all the space available to it. Why then doesn't the atmosphere go off into space?

A) It does, but the rate is so slow that our atmosphere will remain for millions of years.
B) The atmosphere is held down by gravity.
C) The rate at which air molecule escape into space is about equal to the rate that they are produced by various mechanisms, such as volcanic activity and photosynthesis.
D) There are molecular attractions between air molecules and the earth's surface.