A mixture of $\mathrm{H}{}_2$ and $\mathrm{D}{}_2$ (molar masses 2 and 4) with mole ratio $1:1$ is allowed to effuse through a pinhole. The composition (mole ratio $\mathrm{H}{}_2:\mathrm{D}{}_2$) of the gas that effuses out initially is

1. A$2:1$
2. B$\sqrt{2}:1$
3. C$1:\sqrt{2}$
4. D$4:1$