In 1981 children in the United States spent an averageof slightly less than two and a half hours a week doinghousehold chores; by 1997 they had spent nearly six hours a week.
A.chores; by 1997 they had spent nearly six hoursa week1997年的已经不是1981年的那帮孩子了,不能用they
B.chores; by 1997 that figure had grown to nearlysix hours a week这里用的是分号. 而不是主句从句关系. 所以两个句子是有逻辑关系的平行. 1997 并不是过去(1981) 的将来. 既然是平行概念1997 & 1981 都是现在的过去. 而by 有到....时间的意思. 所以发生在1997的过去. 有就是和1981的过去是平行概念的过去. 所以had grown
C.chores, whereas nearly six hours a week werespent in 1997逻辑不对,一周6小时,这是一个频率,不能were spent
D.chores, compared with a figure of nearly six hours a week in 1997
E.chores, that figure growing to nearly six hours a week in 1997
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