题目

阅读理解 My name is Luo Teng. I'm thirteen years old. There are two photos on the wall in my room. They are my favourite photos. In the first photo are my parents and me. We stand in front of a new shop. There are two books in my hands. They are for my birthday. They are about the tiger, my favourite animal. What's in the second (第二的) photo? I work on the computer in my room. I have two computer science lessons every week. They are interesting what's the white animal on my right in the photo? It's my dog. It's my dog It's cute. （1） Luo Teng is years old. A . 11 B . 12 如图，△ABC中，∠C=90°，AC=15cm，BC=20cm．点D从点B出发沿线段BC向点C匀速运动，点E同时从点A出发沿线段AC向点C匀速运动，速度均为1cm/s．当一个点到达终点时另一个点也停止运动．连接DE，设点D的运动时间为t（s），△CDE的面积为S（cm2）．（1）求S与t的函数关系式，并写出t的取值范围；（2）t为何值时，S等于△ABC的面积的一半？（3）将线段DE绕点E逆时针旋转45°，得到线段D′E，过点D作DF⊥D′E，垂足为F，连接CF．在点D、E运动过程中，线段CF的长是否变化？若不变，求出其值，若变化，求出它与t的函数关系式．