Occurrence: Helium comes from natural gas, deposits within Earth, and can be isolated from air.

存在：氦存在于自然大气中，在土壤中沉积，而且能从空气中分离出来。

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At some point later, through natural occurrence or human intervention, that tree became horizontal.

稍晚的时间，因为自然过程或者人为干涉，树倒了。

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Mimicry arises as the result of evolution through natural selection and the occurrence of random genetic mutations that lead over many generations to the appearance of favorable characteristics.

拟态是无数代自然选择和遗传变异的进化而获得的有利特征的结果。

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The occurrence of natural disasters over the last couple of years has increased awareness of the need for data protection and disaster recovery.

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The EAW said the heat-related deaths were a natural occurrence, not uncommon, which started in Mid May.

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The mutation is a natural occurrence, as is the reaction to the drug.

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