选择方框里的单词并用其恰当形式填空，使短文完整，句意通顺。

I am a little fish. I live in a small river. It's dark there, but I am not   1  . There are many fish living near me and I have a lot of friends. We often play   2   together. I feel happy that I am a fish because I love swimming. My mother often tells me something about the sea. I hope I can swim to the sea one day. My mother is worried about me. She often warns me to be   3  . I know she is right. Mother loves me and I love her, too. These days the water of the river is   4 than before. People often throw the rubbish into the river. It's dangerous to live here. We have to  5   to the clean water, maybe to the sea.