题目

________ a precious diamond necklace for his wife, Martin could hardly afford a tie for himself．       A． Having bought                                     B． Buying                                           C． Being bought          D． To buy 答案：A“神舟九号”飞船的返回舱下落到地面附近时，打开降落伞做减速运动．在减速下降的过程中，返回舱的A.动能增大，势能减小，机械能不变B.动能减小，势能减小，机械能减小C.动能不变，势能减小，机械能减小D.动能减小，势能增大，机械能不变