题目

阅读下面材料, 根据其内容和所给段落开头语续写两段, 使之构成一个完整的故事。One time, the teachers at a school wanted to teach the students aboutairplanes. While all airplanes can fly, some are able to fly farther thanothers. This is because not all airplanes are built the same. For example, afighter plane looks very different from a plane that people fly in when theywant to go on a holiday. The teachers wondered how they could make studentsunderstand this. Then, the science teacher, Mr Moose, decided that the school should have apaper airplane contest. Every s12．如图，∠B、∠C的平分线相交于F，过点F作DE∥BC，交AB于D，交AC于E，那么下列结论：①△BDF、△CEF都是等腰三角形； ②DE=BD+CE；③△ADE的周长为AB+AC；④BD=CE．其中正确的是（　　）A．③④B．①②C．①②③D．②③④