"On their back" or "on their backs"?

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Music by Eric Matyas
https://www.soundimage.org
Track title: Dream Voyager Looping

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Chapters
00:00 &Quot;On Their Back&Quot; Or &Quot;On Their Backs&Quot;?
00:20 Accepted Answer Score 10
00:42 Answer 2 Score 5
01:21 Answer 3 Score 0
01:49 Thank you

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Full question
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Tags
#grammaticalnumber #agreement

#avk47