In order to express that something is only one thing from within a larger category, the construction
~は~の一(ひと)つだ will be used. In this expression, (A), the thing that there is one of, will be followed by
は. After this, (B) +
の一(ひと)つだ will highlight the broader category to which (A) belongs.
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アンドロイドはスマートフォンのOSの一(ひと)つだ。
Android is one of (many) smartphone operating systems.
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電車(でんしゃ)は乗(の)り物(もの)の一(ひと)つです。
Trains are one of (many) vehicles.
In this grammar point,
の一(ひと)つ is a standard counter. This means that other counters can and will be used, when they are more appropriate than
一(ひと)つ.
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彼(かれ)も家族(かぞく)の一人(ひとり)だ。
He is also one member of my family.
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トマトはフルーツの一種(いっしゅ)だ。
Tomatoes are one of many types of fruits.
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このクレヨンはこのセットの一本(いっぽん)だ。
This crayon is part of this set.