平流层大气包含臭氧层，臭氧层由紫外线照射导致的三个氧原子结合成的臭氧分子组成。
The still air of the stratosphere contains the ozone layer, which was created when ultraviolet light caused trios of oxygen atoms to bind together into ozone molecules.
外因遗传是一种可以传给下一代细胞的基因调控过程，最常见的方式是通过甲基化作用来完成，该作用可以让甲基（由一个碳原子和三个氢原子组成）附着于腺嘌呤或胞嘧啶上。
Epigenetics is a type of gene regulation that can be passed from a cell to its daughters. The most common mechanism is methylation.
锂原子是由三个电子、三个质子和四个中子组成 的。
The atom of lithium is made up of three electrons, three protons and four neutrons.
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