In 1981 children in the United States spent an averageof slightly less than two and a half hours a week doinghousehold chores杂务; by 1997 they had spent nearly six hours a week.
A.chores; by 1997 that figure had grown to nearlysix hours a week （因为是在1997年以后说的题目当中的话，所以可以用had grown?）
B.chores, whereas nearly six hours a week werespent in 1997