W有用=Gh=10N×0.1m=1J，从图可知，S=2h=0.2m，W总=FS=6N×0.2m=1.2J，W额外=W总-W有用=1.2J-1J=0.2J，η= W有用 W总 = 1J 1.2J =83%．故选A．
