Doubt- In the below example, are we not having pronoun ambiguity by having it at the end, i know it's not underlined but is it correct ?

Why is option C incorrect?

According to public health officials, in 1990 Arizona 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

“It” here clearly has a clear logical antecedent- “age of thirty”

The comparison is- More babies were born to women under the age of 30 than over it. 

In option C- what does “they” stand for- “women” or “babies”- ambiguous.