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>>(*) The only correct answer to "who forgot to turn off the light" is "everybody", because if at least one person had remembered, the light would have been turned off.
>
>It's not correct answer at all.

"Everybody" is sure overrated. More precise answer would be "all of the suspects", i.e. "every one of those who could/should have, but didn't". And the people intuitively go for this restricted meaning.