A boy moving with constant acceleration from A to B in the straight line AB has velocities vA and vB at A and B respectively. Time taken by the boy to move from A to C is twice of that from C to B, where C is the mid-point of AB. The ratio of vA to vB is
A1 : 2
B1 : 3
C1 : 5
D1 : 7
how???