题目

Mobile phones have become a problem for middle schools; Some middle schools in Australia have banned（禁止）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, teachers said mobile phone use is a distraction（娱乐）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 trie下列哪项与毛细血管特点不相符合?（     ） A、数量多，分布广          B、管腔内有瓣膜防止血液倒流 C、管内血流速度很慢        D、细胞只能单行通过