middle schools in Australia have banned（禁止）students from carrying mobile phones to school.

　　Mobile phone use among children has become a problem for the school this year. Several children have got mobile phones as Christmas gifts（圣诞礼物）, and more students want them.

　　Mary Bluett, an official（官员）, said mobile phone use is a distraction（分心）to students during school hours and it also gives teachers so much trouble in their classrooms. Teachers were also saying that students might use phone messages（短信） to cheat（作弊）during exams.

　　She said some schools had tried to ban mobile phones. Some parents felt unhappy because they couldn't get in touch with（和-联系）） their children.

　　Many teachers said students should not have mobile phones at school, but if there was a good reason, they could leave their phones at school offices. They also said there were many reasons why the students should not have mobile phones at school: they were easy to lose and were a distraction from studies.

　　Many people say that they understand why parents would want their children to have phones, but they think schools should let the students know when they can use their phones.

28. Some middle schools in Australia have banned students from carrying mobile phones ____________.

A. because they are students B. when they are at school

C. because they are young D. when they are free

29. We know from the passage that some children get mobile phones from ____________.

A. the makers and sellers B. some other strangers

C. their parents and friends D. some mobile phone users（使用者）

30. Some parents felt unhappy because they couldn't during school hours.

A. get in touch with their children B. leave their mobile phones

　 C. help the teachers with their work D. use their mobile phones

31. The underlined word "they" in the fifth paragraph refers to（指的是）___________.