The Mass of a Deuterium (Diatomic) Molecule Is Roughly the Same

Multiple Choice

The mass of a deuterium (diatomic) molecule is roughly the same as that of an ordinary helium (monatomic) atom. If deuterium gas is at the same temperature as helium gas, the rms speeds of the deuterium molecules is

A) larger than the rms speed of the helium atoms.
B) approximately the same as the rms speed of the helium atoms.
C) smaller than the rms speed of the helium atoms.
D) unknown; more information is needed to work out the answer.