When the gravity of the air discharged by itself is greater than its own gravity, that is, when F floats > G, the hot air balloon floats;

When the heating is stopped, the temperature of the gas in the balloon decreases, the volume decreases, the density increases and the buoyancy decreases.

When the gravity of the air discharged by itself is less than its own gravity, that is, F floats < G, the hot air balloon descends.

Therefore, the hot air balloon adjusts the flying height of the hot air balloon by adjusting the flame size and changing the air density in the hot air balloon, and then selects the required wind direction.

So choose C.