在水中大多数氢原子和氧原子是稳定的，但这两个元素也有微量以较重的同位素形式存在。
Most hydrogen and oxygen atoms in water are stable, but traces of both elements are also present as heavier isotopes.
叶酸是认为在食品中的天然维生素形式。
Folate is the naturally occurring form of the vitamin that is found in food.
这种特殊形式的维生素D天然存在于母乳中。
This special form of vitamin D is naturally present in breast milk.
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