In _____ 1960s, people living along the River failed to keep _______ natural balance of the area. [ ]

A. /; the

B. the; a

C. the; the

D. /; a

滑轮组既能省力又能改变力的方向，建筑工人用自制的滑轮组把600N的重物缓慢提升2m(如图所示)，绳自由端的拉力F=250N，拉力做的功是________J，该滑轮组的机械效率等于________。