If same moles of $O_2$ diffuse in 18 sec and same mole of other gas diffuse in 45 sec then what is the molecular weight of the unknown gas?

Question

$\begin {array}{1,1}(a)\;\large\frac{45^2}{18^2}\times32\\(b)\;\large\frac{18^2}{45^2}\times32\\(c)\;\large\frac{18^2}{45^2\times32}\\(d)\;\large\frac{45^2}{18^2\times32}\end {array}$