You are floating along in a hot air balloon. You look up and notice that the bottom of the balloon is open. Neglecting any diffusion of the molecules, hot air remains inside the balloon despite this opening because a. hot air has more inertia than cold air and doesn't accelerate easily
b. the propane burner located below the opening keeps pushing the hot air back into the balloon
c. hot air has a lower pressure than cold air, so hot air is drawn into the balloon by the partial vacuum inside it
d. the air pressure inside the balloon's opening is the same as the air pressure outside the opening