题目

It is                that he hasn’t               . A. obviously; told the truth                                    B. obvious; told a truth          C. obvious; told the truth                                      D. obviously; told truth 答案：C用如图所示的滑轮组来拉动物块，在2s内使物体移动了4m，已知物体与地面间的摩擦力f为1200N，若此时滑轮组的机械效率为80%（1）求拉绳子的力F为多大；（2）求拉力做的功和功率；（3）如果想用更小的力拉动物体，请你提出一种可行性建议．