路人帮忙－gwd语法题

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.

Any NN can help me with this? I don't know why this answer is A instead of B? And any NN could help me to break down this sentence,whereby analyzing it.

B,C不如A简洁,且B中they精确指代逻辑含义不合理,

D,E时态错误且D使用there不好.

这里是women over 30和women under 30比较。当然只能A。

B的话， born under 30，谁born呢，根据前面，应该是babies born，如果这样 babies born under 30，显然不对了

欢迎大家指正