A force `F = - kx^(3)` is acting on a block moving along x-axis. Here, k is a positive constant. Work done by this force is
A. positive in displacing the block from x = 3 to x = 1
B. positive in displacing the block from x = - 1 to x = - 3
C. negative in displacing the block from x = 3 to x = 1
D. negative in displacing the block from x = - 1 to x = - 3