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のひとりが

one of

→ の
phrase Rare #30,586
one of Used after a group or category to refer to one person in it; が marks that person as the subject.
JA: 生徒のひとりが質問しました。
EN: One of the students asked a question.

連中のひとりがこれを？

One of them did this?

— ドライヴ (2011)

若い男と女子学生のひとりが車にのっていた。

A young man and one of the female students were riding in a car.