A balloon does not rise indefinitely because :
(a) the density of air increases with altitude
(b) the density of air decreases with altitude
(c) the weight of the displaced air increases.
(d) None of the above

(b) the density of air decreases with altitude

Reason — A balloon does not rise indefinitely because the density of air decreases with altitude. Therefore, as the balloon gradually goes up, the weight of the displaced air decreases. It keeps on rising as long as the upthrust on it exceeds its weight. When upthrust becomes equal to its weight, it stops rising further.