In 1981 children in the LACE w:st="on">United StatesLACE> 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

这道题目应该选什么呢？答案是 B

但是为什么要用过去完成时呢？如果可以用过去完成时的话，A也比B好呀，对称，又是主动。