请教个GWD,对时态的选择是在不理解！！！

rudder86

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 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 were spent in 1997
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

答案是B，但怎么也不明白怎么用了个过去完成时？？请各位不吝赐教，多谢多谢

iwvers

C呢，语法中首先被动语态就不优选，其它的我还说不清楚

rudder86

谢谢热心解答，我再好好理解一下

zongzeng90

By 1997表示是在1997年以前就达到6个小时了，所以用过去完成时

waixiaoxi

对时态把握也很弱。 同问。
另外为什么C不可以？