Answer
$ a.\qquad$ less than
$ b.\qquad$ III is best
Work Step by Step
The rms (root mean square) speed of the molecules in a gas at the Kelvin temperature $T$ is
$v_{\mathrm{r}\mathrm{m}\mathrm{s}}=\sqrt{\dfrac{3kT}{m}}=\sqrt{\dfrac{3RT}{M}}\qquad( 17- 13 )$
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$a.$
Interpreting the formula, with temperature being equal,
$v_{\mathrm{r}\mathrm{m}\mathrm{s}}\ \ $is inversely proportional to the square root of mass.
Oxygen molecules, having more mass, have smaller $v_{\mathrm{rms}}.$
$b.$
I is incorrect, as we found the speed to be smaller.
II is incorrect because temperature relates to kinetic energy, not speed alone.
III is best.