根据图示可知,n=2,自由端移动的距离:s=2h=2×2m=4m;A、自由端移动的速度:v= s t = 4m 20s =0.2m/s,故A错误;B、滑轮组的机械效率:η= W有 W总 ×100%= Gh Fs ×100%= G 2F ×100%= 20N 2×12.5N ×100%=80%;故B正确;C、拉力做的功:W总=Fs=12.5N×4m=50J,故C错误;D、拉力做功的功率:P= W总 t = 50J 20s =2.5W,故D错误.故选B.