阅读理解
She is a cute, quiet girl. As a daughter, she has no secrets from her mother, who is very pleased with her. But these days she has become not so open as before. She has a diary that she keeps under lock and key. Her mother cannot help worrying about her: What will I do if she falls in love, which is too early of her age? After all she is reaching the "dangerous time". These thoughts have caused trouble in the mother's mind.
One weekend the girl came to tell her mother that she was going to the cinema with her schoolmate and would return late. This was the first time her mother agreed, and she couldn't help worrying because her daughter had never been away at night before. The mother waited till nine. She decided to go out to meet her daughter. Just at that time, she heard the noise of a car parking. She went to the window and saw her daughter waving goodbye to a boy. Her heart missed a beat.
When the girl came in, the mother was watching TV, just like nothing had happened. "Mum, I'm going to bed."
"All right. Go to sleep early."
The next morning when the mother went to the daughter's room to do some cleaning, she found her diary left on the bed. After a few minutes' thinking, she at last opened it to the page of the night before. It reads: Mum, it's love that made you ask, but it would show your understanding of me if you hadn't.
根据短文内容，选择最佳答案。
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分别为等差数列的第1、4、6项，则这个等差数列前n项和的最大值为