A particle is moving along x-axis back and forth in a box of length L. Assuming de-broglie hypothesis applicable for the particle, and it is moving with constant speed in the box making perfectly elastic collisions to the walls. The possible value of momentum of the particle is : (Note : h is plank constant, n is principal quantum number)
(A) 2 nh/ L (B) nh /L (C) 4 nh /L (D) 2 /3 nh /L