You Are Floating Along in a Hot Air Balloon

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 molecules, hot air remains inside the balloon despite this opening because

A) hot air has a lower pressure than cold air, so hot air is drawn into the balloon by the partial vacuum inside it.
B) hot air has more inertia than cold air and doesn't accelerate easily.
C) convection naturally would make hot air rise, so it has no opening through which to escape.
D) the propane burner located below the opening keeps pushing the hot air back into the balloon.