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.