plural form, another aspect!

[muaxuan68]

Hi all,

let's discussion this sentence.

The most abundant phosphate mineral, apatite, includes several types that vary in their content of fluorine, chlorine, or hydroxyl ions.

The question is why "content" is not in the plural form. what are the general rules?

[Erin]

There is no simple answer to this question. This is the kind of thing that graduate students of linguistics study. But I don't think that TOEFL will underline this part of the sentence.