题目

 阅读理解 请认真阅读下列短文, 从短文后各题所给的A、B、C、D四个选项中, 选出最佳选项。 Everybody hates rats. But in the earthquake places of the world – Japan, Los Angeles, Turkey – rats will soon be man’s new best friends. What happens after an earthquake? We send in rescue dogs. Why? Because they can smell people. Dogs save lives. They help rescuers to find living people. But dogs are big and they can’t get into small spaces. So now a new research project is using a smaller animal to save lives: the rat.     How does it work? First, the 工人师傅利用如图所示的滑轮组匀速提升重990N的货物，所用拉力为600N，则此滑轮组的机械效率为______。