Physics question related to force and acceleration

A force F is applied on a block kept on a frictionless plane for t sec, then another force of same magnitude is applied in opposite direction, then the block will come to rest
(a) immediately (b)after 2t sec from start (c) in a time greater than 2t second (d) in a time less than 2t sec

the answer is (b)

Since F=ma and the force, F, is the same magnitude both ways, the accelerations, a, has the same magnitude both ways.

The acceleration is the rate of change of the velocity. If the rate at which the velocity increases is the same as the rate at which it decreases, it will take the same time to go from zero to the top speed as it takes to go from the top speed back to zero.