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.
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
正确答案为A
请问是it指代了 babies were born to women ? 好像it不可以指代句子。
还是more...than...结构如果主语相同可以省略?
应该怎样解释这道题呢?