please help me with gas laws in chemistry

Question

Explain why a helium weather balloon expands as it rises in the air. Assume that the temperature remains constant.

A. The atmospheric pressure outside the balloon increases, and the gas inside expands to equal that pressure.

B. Pressure and volume are directly proportional at constant temperature.    

C. The atmospheric pressure outside the balloon decreases, and the gas inside expands to equal that pressure.

D. The number of moles of gas increase as the balloon rises.

Answer 1

C: As the balloon rises there is less atmospheric pressure on the outside of the balloon, so it expands to equalize pressure; as pressure decreases, volume increases for the same temperature and moles of gas.