Diffusion is that property by virtue of which a perfume bottle when opened up in a room, makes the whole room fragrant with its smell. If a perfume $\text{'}$X$\text{'}$ diffuses twice as fast as another perfume $\text{'}$Y$\text{'}$; what is the molecular weight of $\text{'}$Y$\text{'},$ if the vapor density of gas $\text{'}$X$\text{'}$ is $2?$ Molecular weight of gas $\text{'}$X$\text{'}$ is to be assumed to be $2.$

$2$

$4$

$8$

$16$