题目

 ________ is often the case with old people, my grandfather is fond of talking about good old days. A. Which                    B. That                     C. What             D. As 答案：D下列关于空气的说法中正确的是A. 拉瓦锡用定量的方法研究了空气的成分 B. 空气中氮气体积分数约为21%C. 空气质量指数越高说明空气质量越好 D. 洁净的空气是纯净物