Dear Daming，

    Thanks for your letter and the wonderful photos. I like them very much.

Here are some of my photos. I'm sending them to you . In the first photo，it is snowing. I'm making a snowman with my friends. In the second one，the sun is shining and the wind is blowing. I'm flying a kite in the park. In the third one，I'm swimming in the sea. In the last one，I am with my sister Lucy. We're playing chess.

Best wishes to you and your family.

Love，

Jack

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地球上最大的生态系统是（  ）

读“我国西部各省区土地及草地退化状况比较图”,回答下列问题。