根据短文内容，完成任务。

      My name is Chen Lan. My home is in Gulangyu. Where is Gulangyu, do you know? It is in Xiamen. It is near the sea (海). Gulangyu is a small place(地方), but it is very nice and clean. There are no cars, buses or bikes. People only walk. So it is very quiet (安静的). My house is in the center of Gulangyu. Behind my house there is a big old tree. My grandfather tells me the tree is very old. There are many birds in the tree. We call it a “bird tree”. My house is near the sea. The sea is big and blue. There are many fish in the sea. After school, I often go there and catch fish with my friends. It is very interesting. I like eating and catching fish.

已知$\left\{a_n\right\}$为等差数列，若$a_3+a_4+a_8=9$，则$S_9=（ \; ）$

“天坑”是分布在喀斯特地区的地貌景观，因地下溶洞的顶部坍塌裸露出地面而成。左图为“天坑”在我国的分布示意图，右图为甲天坑等高线地形图。读图，回答问题。