题目

如图所示，物体A被压在两面墙壁之间，保持墙壁对A的压力不变，当A的重力为20N时，用一个竖直向上的拉力F恰好能使A物体保持匀速向下运动．若A物体的重力减少4N时，用同样大小的竖直向上的拉力F恰好能使物体A匀速向上运动，则物体A上升时，受到两面墙壁总的摩擦力为______N，受到摩擦力的方向________．    We recycle(回收利用)rubbish, so why not language? Recycling language means using vocabulary that you have learnt before again and again. However, unlike rubbish that piles up(堆积)in our environment. if we do not recycle language, we simply forget it. So how do you recycle language? There are basically three ways to do it. The first is through reading. Go back to your favourite articles and read them once more. Just reread them and do not worry about each vocabulary item. If there are some that you are not sure about, make a guess and you will probably guess right. The second way to recycle is through writing. Look at some of your favourite articles and write either a short paragraph or a few sentences in your own words. After you have done this a few times, go back and read through what you have written, checking vocabulary functions. Don't worry if you have made mistakes. That s how you learn. The third way to recycle language is by having an imaginary chat with yourself about your favourite articles at home. You can pretend there is someone with you who speaks English and tell him what you are thinking about or how you feel. 57. According to the passage, recycling language means _______.      A. repeating vocabulary at times      B. revising vocabulary at a proper time      C. learning new vocabulary as much as possible      D. using vocabulary that we have learnt very often 58. If we recycle language, it may _______.      A. be kept in our mind                  B. be forgotten easily      C. pile up in our environment             D. disappear from our mind soon 59. Which of the following is NOT mentioned in recycling language?      A. Reading.       B. Listening.         C. Writing.        D. Speaking. 60. Which of the following is probably the best title of the passage?       A. Why to Recycle Language            B. How to Learn Vocabulary       C. How to Recycle Language            D. Why to Learn Vocabulary