求助答疑

sunnychen123

一道语法题，怎么也看不明白
In 1981 children in the United States spent an average of slightly less than two and a half hours a week doing household chores; by 1997 they had spent nearly six hours a week.

A.chores; by 1997 they[CH1] had spent nearly six hours a week

B.chores; by 1997 that figure had grown to nearly six hours a week

C.chores, whereas nearly six hours a week[CH2] were spent in 1997

D.chores, compared with[CH3] a figure of nearly six hours a week in 1997

E.chores, that figure growing to nearly six hours a week in 1997[CH4]

[CH1]不能指代81年的children


[CH2]放在这里变成了ratio，是格单数，用被动不好


[CH3]后面得是人，与前面children比较


[CH4]by 1997

答案是B 但是我认为此选项用了完成时，没有道理啊。
请高手给个解释

宫小明

By 就是完成时的标志词啊，而且1997是过去的时间，所以就是要用过去完成时。