In response to
nonameee's questions,
This is a great series of questions.
1. When I see 'than', do I have to think of parallelism or not (I believe than will be used in parallelism associated with comparisons?
In general, the word "than" implies a comparison, and a comparison is a special case of parallelism. So, yes, if you see the word "than", your antennae should be up for parallelism.
2. If so, how do I know which parts should be parallel (or in other words, how do I know what constitutes the 'left part' and what constitutes the 'right part')?
I'm afraid to say: what you are asking here is an impossible question. There's no way to give a simple formula that can encapsulate every possible form of parallelism. There are just too many possibilities. The English language has enormous flexibility.
Mostly, it takes practice, and developing an ear for correct English. Reading will help. All the GMAT test prep material will help. My company, Magoosh, has a series of video lessons about parallelism and comparisons --- you can follow the link at the bottom to learn more. Strive to learn as much as you can from each sentence.
3. (B) is wrong because 'born' implies that the babies were born under the age of thirty. Is it also wrong because of bad parallelism?
Think about the sentence.
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.
What is the comparison? More babies were born to women over the age of thirty than
babies born to women under
the age of thirty. English is flexible enough that we can drop all the words in the second clause that are repeated. All the red words are repeated. The only word that's new in the second clause is the word "under", but saying . . . .
... more more babies were born to women over the age of thirty than under.
. . . is awkward. It's awkward to end a sentence with a proposition, and it's awkward that the preposition has no object. So, we insert a simple pronoun:
... more more babies were born to women over the age of thirty than under it.
The antecedent of the pronoun is perfectly clear. This sentence is 100% grammatically correct as is.
The first problem with (B)-(E) is that, when you already have a 100% grammatically correct option, these others, grammatically correct or not, are all
longer. If you have two options, both grammatically correct, and one is shorted than the other, the short one will be preferable on the GMAT 100% of the time.
Furthermore, with choice (B),
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 born under it.
Misplaced modifier! In the first clause, it's clear that "over the age of thirty" modifies "women" --- the modifier "touch" rule. If we just omit and imply all of that, as we do in part (A), all the relationship that were correct in the first clause are implied in the second clause. In (B), we shove the word "born" into the second clause, and now it sounds like "under it" (i.e. "under the age of thirty") modifies "born" --- if you touch a word to a modifier, you are asking the modifier to modify it! So yes, it sound like we are talking about all those babies that are under they age of thirty when they are born (which would be 100% of them!). We are not comparing mothers over 30 to babies under 30. That's the problem with (B).
There is something very clean about omitting every word that would be repeated from the first clause: all the correct grammar in the first clause is simply implied in the second clause without having to say it.
I hope that helps. I realize, especially with the second question, it's not a complete answer, because a complete answer would be impossible to articulate. FWIW, I do think you would find the Magoosh series of grammar videos helpful.
Here's a free SC question on parallelism.
https://gmat.magoosh.com/questions/1115
The question at that link should be followed, after you submit your answer, by a complete video solution.
Let me know if you have any further questions.
Mike
