清明节前的一个晚上，作者漫步在____，听到一声____，陷入了对往事的回忆——1947年初秋，激烈的围歼战打响了。在____的时刻，郝副营长____一本书，为后续部队照亮了前进的路却牺牲了自己。很久以后，在天安门璀璨的华灯下，又一次____。

    Do you know about Xiong'an? It's a new city near Baoding. There is a big lake in Xiong'an. Yes, it is Baiyangdian Lake. It is very big. In summer, you will see many beautiful lotus flowers on the lake. Some are red. Some are pink. Many people like them. There will be many boats on the lake. And there will be many birds singing in the grass. Xiong'an is beautiful and new. You can come here by bus from Baoding. And you can come here by train. You will play in the water with your friends. You will eat delicious fish. You will take many wonderful pictures. You will have a nice trip in Xiong'an.

一个正多边形的每个外角都是72°，这个正多边形的边数是（　　）