题目

My wife passed away a few years ago, and I went through the worst time in my life． I even wanted to kill myself． Just for kids, I had to continue to live and work as small-town doctor at my medical clinic in Hawaii． My kids had gone to live on the mainland, and I was alone． Then they asked me to have a family trip． On our trip, we turned on the TV at the motel and saw the second plane crash into the World Trade Center． Seeing it falling down, I said to my kids: “I’m going to Afghanistan”． And a few weeks later, international Medical Corps sent me to set up 20 clinics如图示，拉滑轮组的自由端能使物体向上升．要提高滑轮组提升重物的机械效率（忽略摩擦力），以下方法可行的是A.换用一个较重的动滑轮B.换用一个较重的定滑轮C.换用一条较重的拉绳D.增加所挂重物的重量