GMAT Sentence Correction (SC) | In 1981 children in the US spent an average

In 1981 children in the US spent an average of slightly less than 2 and a half hours a week doing household chores;by 1997 they had spent nearly six hours a week.




Could some one please explain by eliminating the answer choices and the reasons for the elimination. ( A shout out to the experts)

Thank you,
Shiv

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