题目

阅读理解 Reading can be a social activity. Think of the people who belongto book groups. They choose books to read and then meet to discuss them. Now, thewebsite Book Crossing.com turns the page on the traditional idea of a book group. Members go on the site and register the books they own and wouldlike to share. Book Crossing provides an identification number to stick inside thebook. Then the person leaves it in a public lace hoping that the book will have an adventure, travelingfar and wide with each new reader who finds it. Bruce Pederson, the managing director of BookCrossing, says,The两种滑轮的特点：定滑轮的特点：使用定滑轮    省力，但    改变动力的方向．（能或不能）动滑轮的特点：使用动滑轮    省力，但    改变动力的方向．（能或不能）