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Which one is standard?

"Everybody took their seat."
"Everybody took their seats."
"Everybody took his seat."

How about if I changed the subject to "All the guests"?

Should it be
"All the guests took their seat."
or
"All the guests took their seat."

Thank you.
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lagatawWhich one is standard?

"Everybody took their seat."
"Everybody took their seats."
"Everybody took his seat."
Purists insist on "his": Everybody took his seat. Ordinary people use "their", with either the singular or the plural. I would use the plural. Everybody took their seats.
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lagatawShould it be
"All the guests took their seat."
or
"All the guests took their seat."
They're both the same. Maybe you meant to make one plural. I would use the plural, as above. All the guests took their seats.

CJ
Comments  
If you use "everybody", so the posesive is his or her because it is singular.
"All the guest" is plural, so you can use "their"
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 CalifJim's reply was promoted to an answer.
CalifJim"All the guests took their seat."
They're both the same. Maybe you meant to make one plural. I would use the plural, as above. All the guests took their seats.

Yep, that's what I meant. Thanks CJ for always giving attention to my questions.