题目

During the last winter holiday, I went to China with my parents. The five-day trip left me with a deep impression.As the capital of China, Beijing is a very beautiful city with fresh air and clean streets which make the travelers feel very pleased. To my surprise, many people there were learning English. Later I was told that they did so because Beijing would hold the 29th Olympic Games and they wanted most foreigners to understand them. They strictly kept the traffic rules. When there was a red light, no one was seen crossing the street.Of all the places I visited, I liked the Summer Palace b有经验的鱼民在进行长途运输鲜鱼的过程中常常在水中加入少量的过氧化钙固体．已知过氧化钾的化学式可表示为K2O2，则其中氧元素的化合价为    ．则过氧化钙的化学式为    ．若过氧化钾与水反应的方程式表示为2K2O2+2H2O=4KOH+O2↑，则过氧化钙与水的反应方程式    ．请说出使用过氧化钙可以使鱼长期存活的理由：    ．