The end of a chain of length L and mass per unit length `rho`, which is piled up on a horizontally platform is lifted vertically with a constant velocity u by a variable force F. Find F as a function of height x of the end above platform:
A. `rho (gx + 2u^(2))`
B. `rho (2 gx + rho u^(2))`
C. `rho (gx + u^(2))`
D. `rho (u^(2) - gx)`
