$(B)$ $\rightarrow$ R.Ms. velocity $=\sqrt{\frac{3 R T}{M}}$, So it depends on Mass olso. So two different gases will have different R.M.S velocities. So option $B$ is incorrect.

$(C)$ $\rightarrow$ Similar as above option B. R.M.S depends on molar mass also. So. option $C$ is ircorrect.

$(D)$ $\rightarrow$ No. of molecules $=\frac{\text { mass }}{\text { molar mass }}$ So, no. of molecules is inversely paportional to molar mass. The molar mass of oxygen molecules are more, so No. of molecules of oxygen will be less. option. $D$ is incorrect.