The density of a gas $\text{'}$X$\text{'}$ is twice that of another gas $\text{'}$Y$\text{'}.$ If the molecular weight of gas $\text{'}$Y$\text{'}$ is $\text{'}$M$\text{'}$; then the molecular weight of the gas $\text{'}$X$\text{'}$ will be

$2$M

M$/2$

M

M$/4$