When gas is heated, it expands. Imagine a balloon that has some air in it, but not a lot. Now, if the air inside it is heated, the air expands and stretches out the balloon. According to the kinetic molecular theory theory, what is happening to the air molecules that causes the balloon to expand?

A) The molecules are melting.
B) The molecules are moving faster.
C) The molecules are moving slower.
D) The molecules themselves are expanding.