题目

I had never seen sand dunes（沙丘）before. If you go to the seaside in Britain, you may see some small hills of sand, but nothing very impressive. Of course, Africa is home to many beautiful “sandscapes”, but I was in China, so I went to Dunhuang.    I had spent my first day looking at the beautiful Mogao caves. Now I wanted to experience something very different.    I have been walking in Asia, Europe and America, but nothing prepared me for walking over hills of sand. Whenever I took a step up a hill, my feet would sink into the sand and move backwards. Sometimes it seemed as if 科学实验证明，空气中的微量臭氧（O3）可以抵抗阳光中紫外线对人体的危害，空调和冰箱中的致冷剂“氟里昂”是一种氟氯烃（以M代替），它可以破坏臭氧层，其反应过程为：M+ O3   →MO+ O2      MO+O→M+ O2 A、氧化剂    B、还原剂    C、催化剂   D、稳定剂