海王星是太阳系中唯一一颗先由精确的数学推导预测出来，再通过观察确定的行星。

Neptune is the only planet in our Solar System whose existence and location was mathematically predicted before the planet was actually observed.

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我还亲自摸索到了一块正在清洗之中的钻石——人们告诉我说，这是在美国发现的唯一一颗真正的钻石。

I searched in the washings for a diamond and found it myself-the only true diamond, they said, that was ever found in the United States.

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土卫六一直以来都是这项探测任务的主要聚焦点，因其体积比水星还要大，是唯一一颗拥有浓密大气层的卫星。

Titan was a major focus of the mission all along, since it's bigger than the planet Mercury and the only moon massive enough to have a significant atmosphere.

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