In 1981children 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 week(比较对象不是孩子做作业用多少时间这个动作,而是用了多少时间这个数据,且they表示就是前句的children,但16年后他们怎么能是同样一批孩子呢?所以they错)
B.chores; by 1997 that figure had grown to nearlysix hours a week(1997是过去时间,by +time用完成时——>用过去完成时,虽然1997在1981之后,这里的had grown表示已经,不表示过去的过去)
C.chores, whereas nearly six hours a week werespent in 1997(没必要用被动态;比较对象有歧义)
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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