article: My name is Chen Lan. My home is in Gulangyu. Do you know it? It is in Xiamen. It is near the sea . Gulangyu is a small place,but it is very nice and clean. There are no cars,no buses. People only walk. So it is very quiet.
  Our house is in the middle of Gulangyu. Behind our house there is a big old tree. My grandfather tells me that the tree is very,very old. There are many birds in the tree. We call it a "bird tree". Our house is near the sea. The sea is big and blue. There are a lot of fish in the sea. After school,I go there and catch  fish with my friends. It is very interesting. I like fish and I like catching fish.
question: What is behind our house?
options: a river a sea an old tree fish