The speed of pulling the rope by hand is v2 = 2v 1 = 2× 0.25m/s = 0.5m/s, which is wrong.

B, useful work done by tension w useful = GH = 50n× 5m = 250J j. This option is wrong;

C. The pulling force exerted by hand is f =12 (g+G0) =12× (50n+4n+6n) = 30n.

The length of the hand rope is S=2h=2×5m= 10m.

The total work done by tension is W total = FS = 30N× 10M = 300J. This option is wrong;

D, the mechanical efficiency of the moving pulley η=W, always useful w = 250J300J ≈ 83.3%. This option is correct.

So choose D.