GWD26-Q20 中 that 指代问题

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,

其中that 的指代该怎么讲? that 这种用法好像不少,请高手讲讲!