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阅读短文,选择正确的答案。 Mary is a little girl.She’s only four. She doesn’t go to school.So she doesn’t know how to read or write.Her sister Kate is a schoolgirl. One day,Kate sees her little sister at the desk and there is a pen in her hand and a piece of paper in front of her. “What are you doing,Mary?”she asks. “I’m writing a letter to my friend Ketty.”says Mary “Oh,my dear!But you don’t know how to write!” “Well,”says Mary,“It doesn’t matter,because Ketty doesn’t know how to read,either.” Mary doesn’t know ________.