阅读短文，下列问题

 B

American boys and girls love to watch television. Some children spend six hours a day in school and four to six hours a day in front of the television set. Some even watch television for eight hours or more on Saturday.

     Televisions are like books or films. A child can learn bad things and good things from them. Some programmes (节目) help children to understand (理解)the news, others show people and places from other countries or other time in history. With television a child does not have to go to the zoo to see animals to the ocean to see a ship. Boys and girls can see a play, a concert or a game at home.

     Television brings many places and events into our homes. Some programmes show crime (犯罪) and other things that are bad for children, so parents sometimes help them to find other interesting things to do.

某市团委举办“我的中国梦”为主题的知识竞赛，甲、乙两所学校参赛人数相等，比赛结束后，发现学生成绩分别为70分，80分，90分，100分，并根据统计数据绘制了如下不完整的统计图表：

“魔力蜡烛”可以增加生日宴会气氛，制造蜡烛时，在烛心中掺入镁粉（镁粉在430℃时能自燃）。每次吹灭蜡烛时，余烬会引燃烛心中的镁粉，产生火花，又重新点燃蜡烛。“魔力蜡烛”能“死灰复燃”，说明燃烧的条件之一是____。