The vander Waals' constant 'a' for the gases $CH _{4^{\prime}} N _2, NH _{3}$ and $O _{2}$ are $2.25,1.39,4.17$ and $1.3 L ^{2}$ $atm-mol ^{-2}$ respectively. The gas which shows highest critical temperature is :
(A) $CH _{4}$
(B) $N _{2}$
(C) $NH _{3}$
(D) $O _{2}$