求助GWD-12-Q30

According to public health officials, in 1998 Massachusetts became the first state in which more babies were born to women over the age of thirty than under it.

A.    than

B.    than born

C.    than they were

D.    than there had been

E.     than had been born

答案是A，能否有语法高手告知，这道题目所涉及的比较（more...than）的对象到底是什么，还有句尾的it指代的是什么？

非常感谢！

more babies were born to （women over the age of thirty ）than （women） under it（age of 30）.

参见cwang的精华帖：比较之庖丁解牛

http://forum.chasedream.com/dispbbs.asp?boardID=23&ID=103101&page=1

可以套出选项A是正确的。