导致视力下降的主要原因有两个：花在室内学习的时间太多，花在户外玩耍的时间太少。

Two main reasons can cause poor eyesight: too much time spent indoors studying and too little time spent outdoors playing.

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巧克力短缺的主要原因有两个——巧克力越来越受欢迎，以及制作巧克力的植物可可产量减少。

There are two main causes of the chocolate shortage—chocolate's growing popularity and less production of cocoa, the plant from which chocolate is made.

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这件事很重要，主要原因有两个。

This matters for two main reasons.

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There are two reasons, for the most part, why they are coming: they're coming to find poems that will be relevant, equal to the occasion of a death or a marriage.

主要有两个原因能解释为什么，人们访问美国诗人协会网站：,人们是来寻找一些诗歌，一些有关,死亡与婚姻等的诗歌。

普林斯顿公开课 - 人性课程节选

Another characteristic of these years, which scholars refer to as the Dark Ages, just as they do the years after the fall of the Roman Empire-- dark for two reasons. Dark, in the most obvious way, because we don't have any writing, no record of them.

这也是这段时期的另一个特征,学者们称之为,黑暗时代,他们也是这么称呼,罗马帝国陷落后的那段时期的,称黑暗主要有两个原因,黑暗,最明显的理由,是因为我们没有任何关于他们的史料和记录

耶鲁公开课 - 古希腊历史简介课程节选