如图a是长方形纸带，∠DEF=20°，将纸带沿EF折叠成图b，再沿BF折叠成图c，则图c中的∠CFE的度数是（　　）

阅读理解

    Every country has its favourite food. In some countries, people(人们) eat rice every day. They often eat it with meat, fish and vegetables.       

   Japanese(日本人) eat a lot of fish, and they eat a lot of rice, too. In England(英国), the favourite food is fried fish and French fries. In the USA(美国) people like hamburgers a lot. And they like to have cold drinks. In Canada, people eat bread with butter(黄油) on it. In India, people like to eat hot food. Some people eat only fruit and vegetables. They don't like to eat meat and fish. They think green food is good for them.

把下列各数分别填入相应的大括号内：

−7，3.5，−3.1415，π，0，$\frac{13}{17}$，0.03，$-3\frac{1}{2}$，10，，$-\frac{4}{2}$

自然数集合{                                             …}；
整数集合{                                               …}；
正分数集合{                                             …}；
非正数集合{                                             …}；
有理数集合{                                             …}．