题目

阅读下面短文，根据短文大意和语境选择方框内的单词，并用其正确的形式填入空白处，使补全后的短文意思通顺，语法正确，结构完整。 send     expensive     waste      problem     cool Dear headmaster, I’m a student of Grade 9. I want to tell you something. In our school, many students have mobile phones. They think it’s   1   to have a mobile phone. Yes, the phones let students talk with their parents and friends at any time. But I don’t think it’s good for students to use them at school. Many students spend l函数的定义域为                   ．