Answer
See explanation.
Work Step by Step
If the balloon were filled with air at ambient temperature, the weight of the air displaced would be roughly equal to that of the balloon and its enclosed air, which generates no lift.
It’s necessary to use hot air, which is less dense than cold air, so that the balloon displaces a volume of air whose weight is greater than its own weight. The buoyant force then lifts the balloon and the basket beneath it.
The balloonist can control ascent and descent by heating the air up or letting it cool down, which adjusts the buoyant force.
