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但是b的这个时态我真是搞不懂，请高人指点