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 lots of time in  2   messages by mobile phone, some students even use them to play games in class or cheat (作弊) in the exams. Some students
always hope to buy the new or   3   mobile phones and this will   4   their
parents’ money. And my father tells me that most mobile phones can cause (引起)
many health   5   . Could you do something to stop it?
Yours,
Sophia