题目

阅读理解 If you're a cycling lover, youwill have to pay a lot of attention to your bicycle and maintain it well, for bicycle parts are exposed to wear and tear (磨损) over time and you have to examine the bicycle to see if there are any signs of wearing such as scratches, dents (凹陷) etc. Sometimes, a part maynot work because it has exceededits life expectancy. Sometimes, you will need to find replacement parts for it to keep up the performance of the bicycle. Different parts of a bicycle will wear at different rates so it is very important that you have a basic understanding about t写出反应的化学方程式：用过氧化氢与二氧化锰混合制氧气2H2O2 MnO2 . 2H2O+O2↑2H2O2 MnO2 . 2H2O+O2↑．