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    A man lived in a village. He had an old cat at home. The cat couldn't run fast and bite，because she was so old. One day, when she saw a mouse, she jumped and caught it. But she could not bite it. The mouse got out of her mouth and ran away.

    Then the man became very angry and began to beat the cat. The cat said, “Don't beat your old servant. I know I'm old and can't kill a mouse. But I have worked for you for many years, and I still want to work for you. Be kind to the old, and remember what good work they did when they were young."

    When the man heard that, he realized that he was wrong.

绿色植物通过光合作用维持了生物圈中的二氧化碳和氧气的相对平衡。