GWD-12-Q30:比较题~~help

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

Ans:A

想问下是什么的比较？省略什么了吗？

thanks~