根据短文内容，选择最佳答案。
Mobile phone has become a problem for middle schools. Some middle schools in Australia have banned students from carrying mobile phones during school hours.
Mobile phone use among children has become a problem for the school this year. Several children have got mobile phones as Christmas gifts, and more students will want them.
Mary Bluett, an official, said mobile phone's use is a distraction to students during school hours and it also gives teachers so much trouble in their classrooms. Teachers were also saying that sometimes students might use phone messages to cheat during exams.
She said some schools had tried to ban mobile phones. Some parents felt unhappy because they couldn't get in touch with 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 office. 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（分散注意力） 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 mobile phones.
Z(g)(未配平)。温度为T0时，在容积固定的容器中发生反应各物质的浓度随时间变化的关系如图a所示。其他条件相同，温度分别为T1、T2时发生反应，Z的浓度随时间变化的关系如图b所示。下列叙述正确的是
