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（1）     GWD-26-Q20

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，为什么C不对呢？

我觉得

1， C的被动，没有指明施动者， 被动本身就不太好

2， whereas 在这里有转折的意思， 好象不太符合原文的语境

3， by 1997和 in 1997还是有差别的， by 有个过程的感觉

请指教