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According to my answer sheet, c is wrong, but I don't know why? Can you please explain to me?

a. According to a recent productivity study at Harvard, taking a nap at work may be a productive thing to do.

b. Thirty people in the study were tested four times a day on how quickly they could process information.

c. It was found that the people who napped for half an hour during the day did better than those who stayed awake the whole day.

d. Indeed those who did not take a nap showed a decline of 50 % in their ability to process information.
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I see nothing wrong with C in the text you have presented; indeed, I can find nothing wrong at all except the missing comma after 'indeed' in D.
Comments  
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To me, I think the word 'the' in 'It was found that the people who napped...' is wrongly used.
agree with Danny

The "the" in C should not be there
Sorry boys, but 'the' is perfectly good and in fact preferred there. The passage does not talk about any 'people'; it speaks of the nappers among the 30 in the study. In fact, the definite article is usual with post-specification even without a previously designated group:

The people who came to my party were all over 60.

The people who voted for Obama are not so happy now.

Etc, etc.
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