假定英语课上老师要求同桌之间交换修改作文，请你修改你同桌写的以下作文。文中共有10处语言错误，每句中最多有两处。错误仅涉及一个单词的增加、删除或修改。
增加：在缺词处加一个漏字符号(∧)，并在其下面写出该加的词。
删除：把多余的词用(＼)划掉。
修改：在错的词下画一横线，并在该词下面写出修改后的词。
注意：1. 每处错误及其修改均仅限一词；
2. 只允许修改10处，多者（从第11处起）不计分。
Many young boys and girls had the habit of smoking, though they are middle school student. As we all know, smoking do harm to human beings. More and more people have realized that how serious this problem is. But they are never boring with it. Some people think smoking is a kind of fashion, and other think smoking can refresh himself. In fact, smoking causes many illnesses. The most serious illness led by smoking is lung cancer. Meanwhile, smoking is the waste of money. What's worse, careless smokers may cause danger fires.