The vapour pressure of a pure liquid A is 10.0 Torr at $27^{\large\circ}C$,One gram of B is dissolved in 20g of A,the vapour pressure is lowered to 90 Torr.If the molar mass of A is $200gmol^{-1}$,the molar mass of B is

Question

$\begin{array}{1 1}75gmol^{-1}\\85gmol^{-1}\\100gmol^{-1}\\115gmol^{-1}\end{array} $