They look into wrong mark given to swimmers (1-4)

I believe the answer is:

x-rays

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'they' is the definition.
Both the answer and definition are plural nouns.
Maybe they are linked in a way I don't understand?

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for x-rays that I've seen before include "Electromagnetic waves, for medical diagnoses, say" , "Diagnostic radiation" , "Medical images" , "Hospital photos" , "Medical photographs" .)

I've seen this clue in The Guardian.

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