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I know this is probably a basic grammar question but why is it 'one of the thick tubes that carry blood from the heart to other parts of the body' and not 'one of the thick tubes that carries blood from the heart to other parts of the body'?

Thank you

PBF
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Hi,
The writer intends 'that' to relate to 'tubes' and not to 'one'.

Best wishes, Clive
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It's grouped as:

One of [the thick tubes that carry blood from the heart to other parts of the body].

So "carry" agrees with "tubes".

[One of the thick tubes] that carries blood from the heart to other parts of the body. is, I suppose, theoretically possible, and would mean there are many thick tubes having different purposes, and we're referring to one of those whose function happens to be as stated. If I meant this, though, I would rewrite it to avoid possibly ambiguity or misinterpretation.