GWD26-Q20

coffeeqoo

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

求问正确答案B中时态如何理解？

coffeeqoo

求指点！