一道关于It完全指代的题 请大家帮忙解惑

Between 1990 and 2000 the global economy grewmore than it did during the 10,000 years from the beginning of agriculture to 1950.

A.    Between1990 and 2000 the global economy grew more than it did during the 10,000 yearsfrom the beginning of agriculture

B.     Between1990 and 2000 the growth of the global economy was more than that during 10,000years, from when agriculture began

C.    Thegrowth of the global economy between 1990 and 2000 exceeds that which had beenfor 10,000 years from the beginning of agriculture

D.    Thegrowth of the global economy between 1990 and 2000 exceeds what it has been for10,000 years, from when agriculture began         (A)

E.     Thegrowth of the global economy between 1990 and 2000 exceeded what it did for the10,000 years from the beginning of agriculture

这道题为什么it在A选项中指代的是the global economy,但是到了D选项中指代就成了the growth of the global economy between 1990 and 2000?

想不明白呀><