A, pulling the crown block, no effort, A wrong;

B, the work done by tensile force is: w sum =Fs=250N×0.4m= 100J.

Then p total =W total t= 100J2s=50W, b error;

C and w are useful = GH = mgh = 20kg×10n/kg× 0.4m = 80J.

Then η=W is always useful w×100% = 80j100j×100% = 80%; C is correct;

D. In this process, the mass and speed of the counterweight remain unchanged, the kinetic energy remains unchanged, and the gravitational potential energy increases, so the mechanical energy increases, and D is wrong.

So choose C.