When the air inside a hot air balloon is heated, it expands and takes up a larger volume. as a result, it floats higher into the sky. what else happens to the air inside the balloon as it is heated?
a density decreases.
b density increases.
c mass decreases.
d mass increases.

The mass remains the same given that the number of air molecules remain the same and none did escape into the atmosphere.
As heat is being absorbed by the air molecules, they vibrate more vigorously and the electrostatic forces of attraction between the molecules weaken and they become further apart. Hence there is lesser number of air molecules per volume, indicating that density has decreased.