GWD TN17中的一道语法题

ananfighting

题目：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

答案选B。看到好多解释，说A中的they不能指代children，因为是定向指代，这一点不懂。。

另外，B中的had grown 可以吗？前面的spent是过去时，这个是过去完成时，没有问题吗？

恳请回答，感谢！

287151319

they指代children是1981年的children，到1997年他们就不是children了，这里想比较的是1981年的children和1997年的children。
两个句子时态不影响，前面描述的过去的事件用过去时。后面表示从1981到1997年为止，对于现在来说是过去的过去，出现了两个时间点，所以用过去完成时。