Do you have to feed plants like you feed chickens? Or do they grow up by themselves? by themselves 能否改为 themselves?
在正式语体中，by themselves 是不能用 themselves 来替换的。oneself 用作状语（强调语）时，它与其先行词存在着同位关系，其意义为：“是……而不是别人”“就本人而言”。可译为“亲自”“本人”：
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One always has to do these things oneself. 这些事情都得亲自去做。
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I myself will go. 我将亲自去。
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To do something oneself is often easier than getting someone else to do it. 亲自干一件事往往比叫别人干还容易些。
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She must decide that herself. 她必须自己决定那件事。
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Did you cook it yourself? 是你自己做的吗？
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He writes most of his songs himself. 他唱的歌大部分是他自己写的。
而 by oneself 是指: A. “单独”“独自”（无人陪伴）(alone); B. “独立地”“靠自己”（无人帮助）。
A. 1. They are living by themselves. 他们单独居住。
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One can't play tennis by oneself. 一个人不能独自打网球。
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Well, I bought it, so now you'd better marry me because I can't live in it by myself. (Hillary, Living History) 我已经把这房子买下了，现在你最好嫁给我，因为我不想一个人住在里面。
B. 1. He did it by himself. 他独立地做了这件事。
- Did you draw this picture by yourself? 你是自己画的这幅画吗？
[附注]在非正式语体中,by oneself 中的 by 是可以省略的:
One could do it (by) oneself but it would be difficult. 人们可以独立地做这件事,但是,那将是非常困难的。
请做下面的英语试题：
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Tim is too young ____. (他不能自己一个人出去) (全国成人高考, 94-71)
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Tom felt that he knew everybody's business better than they knew it ____.
A. themselves B. oneself C. itself D. himself(全国高考,96-12)
请分析下面 by oneself 结构的含义并把句子译成汉语：
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By the time he was fourteen years old, he had learned maths all by himself.
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Training by yourself in a gym can be highly dangerous.