There are two taps, first tap fills tank P and second tap fills tank Q. The rate of flow through second tap is twice of that through the first tap. Tank P gets filled up in 4 hrs and tank Q gets filled up in 6 hrs. Find the ratio of the capacities of the two tanks.

a. 2: 3
b. 1 :3
c. 3 : 2
d. 1: 2