more babies were born to women over the age of thirty
than (babies were born to women省略相同的S+V) under it（指代the age of thirty）

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.
据公共卫生官员称，1998 年，马萨诸塞州成为第一个 30 岁以上女性所生婴儿多于 30 岁以下女性（所生婴儿）的州。

A. correct； than后面省略了babies born to women； 因为under与over的平行， it明确指代the age of thirty.(换句话说， 有了Under就能知道than后面省略的是women了，不用像C补出they were了)

B. under it修饰了born，逻辑意思荒谬：孩子们生下来时小于30岁。

C. 根据比较的平行 they 指代 babies，但仅补出 they were 使得 were 是助动词还是系动词不清楚，系动词 的话， under it 修饰了 they，逻辑错误，助动词的话， they were 累赘。
（they 仅指向 babies 而不是指 向 30 岁以上母亲剩下的 babies，类似的例子如： In Japan elderly people are treated with far greater
respect than they are in most Western countries.they 不是指代 Japan 的老年人，而是就老年人）

D. had时态错误，比较双方是同时发生的，应该都用一般过去时； there无并列对象。
E. had时态错误同D