GMAT语法一道。急！

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 weekB. chores; by 1997 that figure had grown to nearly six hours a weekC. chores, whereas nearly six hours a week were spent in 1997D. chores, compared with a figure of nearly six hours a week in 1997E. chores, that figure growing to nearly six hours a week in 1997答案是B。请详细说一下C，D怎么错了，尤其是C。谢谢！