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Which of the following statements best explains why a closed
balloon filled with helium gas rises in air?
(a) Helium is a monatomic gas, whereas nearly all the molecules
that make up air, such as nitrogen and oxygen, are
(b) The average speed of helium atoms is greater than the
average speed of air molecules, and the greater speed
of collisions with the balloon walls propels the balloon
(c) Because the helium atoms are of lower mass than the average
air molecule, the helium gas is less dense than air.
The mass of the balloon is thus less than the mass of the
air displaced by its volume.
(d) Because helium has a lower molar mass than the average
air molecule, the helium atoms are in faster motion. This
means that the temperature of the helium is greater than
the air temperature. Hot gases tend to rise.