Before Boarding an Airplane You Buy an Airtight Foil Package

Before boarding an airplane you buy an airtight foil package of peanuts to eat during your journey. While in flight you notice that the package is puffed up. Explain.

A) The peanuts have expanded much like popcorn expands upon heating.
B) The air molecules within the package are now closer together.
C) There are fewer air molecules pushing against the outside of the foil package.
D) The rate at which air molecules within the foil package hit the inner surface has increased.