游乐场的回转车，第二个圆形轨道要比第一个矮些，之所以要这样设计，其原因是回转车运动时受到____的作用，机械能总量____.

原鸡是家鸡的祖先，家鸡和原鸡有很多相似的特征，但家鸡的产蛋量高，人类能把原鸡驯化成产蛋量高的家鸡，是因为生物有    (    )

      The most valuable gift I have ever received was a photo album from my best friend Kevin.

      Kevin and I were classmates for all six years of primary school. We both had the same interests, so we always had a lot of things to talk about. It is wonderful to have such a good friend.

      However, last summer, Kevin had to move to another city because of his father's job. Then he gave me a photo album. “It has photos of the memorable events we have experienced together. I've also written some notes under each photo,” he said. I was deeply moved.

      I put the album beside my bed. I always look at it when I think of Kevin.