A Weather Balloon Filled with Helium Gas Gets Larger as It

A weather balloon filled with helium gas gets larger as it ascends to high altitudes. The expansion of the balloon is primarily due to

A) a decrease in the average kinetic energy of the helium atoms inside the balloon.
B) an increase in the average kinetic energy of the helium atoms inside the balloon.
C) a decrease in the average kinetic energy of the air molecules outside the balloon.
D) an increase in the rate of collision of the helium atoms against the inside walls of the balloon.
E) a decrease in the rate of collision of the air molecules on the outside walls of the balloon.