Question

What causes pressure inside a helium balloon?
A. The helium atoms stick to the surface of the balloon and increase
its weight
B. The helium atoms expand and press on the surface of the balloon.
C. The helium atoms exert an electrostatic force that pushes the
surface outward.
D. The helium atoms bounce off the surface as they move inside the
balloon.

Answer 1

Explanation:

So the gas pressure of a helium balloon arises from the impact of the collisions of the helium atoms between themselves and with the inside surface of the balloon. Of course, the outside atmosphere similarly exerts a pressure on the outside of the balloon.