Sentence Correction

Hey Guys,
I'm going over my results from my GMATPrep practice test, and for the most part I can reason out the mistakes that I have made. However, there are a few sentence correction Q's that have me scratching my head, mostly because I can't figure out what grammatical rule the question is using (and the test doesn't provide answer explanations to the best of my knowledge). So, I thought I'd call out to this community and see if you guys can help me work my way through these. I've included the sentence with answer choices and my logic for picking the wrong answer (or for avoiding the right answer).

1. According to public health officials, in 1998 Massachusetts became the first state in which more babies born to women over the age of thirty than under it.
A) than (this is the correct answer, but I crossed it out assuming that this was a parallelism issue. I thought, incorrectly it seems, that the verb "born" had to be included after "than" whether implied or explicitly. Is this an idiomatic thing, or am I simply over-thinking this question?
B) than born (if given a second chance this would probably be the answer choice I picked, although it's still wrong).
C) than they were
D) than there had been (this was the answer I chose, although on review even I admit it sounded awkward)
E) than had been born

2.) The number of people flying first class on domestic flights rose sharply in 1990, doubling the increase of the previous year.
A) doubling the increase of (this is the correct answer. my first instinct was to compare the verb tenses in both clauses. when i saw the past tense in the first clause, i figured i needed the past tense in the second clause. therefore, i immediately crossed this answer off, even though i didn't find any good alternatives in the answer choices. i've done a bit of research and it looks like participle phrases are used to modify nouns-in this case, most likely "the number"-and therefore the use of the present participle in this clause is probably ok...can anyone confirm this logic for me?)
B) doubling that of the increase in
C) double as much as the increase of
D) twice as many as the increase in (this was the answer choice i picked, mostly because i eliminated the correct one and forced myself too choose among the remaining flawed choices...even i admit that this choice seems very awkward).
E) twise as many as the increase of

1...more babies born to women over the age of thirty than under it.
It's hard to shrink the size of the given sentence but we gotta do this.
when you write it all out, it goes...
..more babies born to women over the age of thirty than (more babies born to women) under it(=the age of thirty). Therefore (A) is the correct answer.
(B) than born- the subject is missing. I guess we need to have a subject if the verb(born) is present.
(C)than they were--what does "they" refer to? babies? women? the term is vague. One more thing, it's not parallel.
(D)the tense is wrong and the "there" is unnecessary and wordy.
(E)tense and subject is missing. Besides, it's wrong on all levels.

2.Let's start iwth (B)
(B) doubling that of the increase in-It is a wise choice to use <~ing> which describes/modifies a clause. <that> is needed when it describes/modifies <noun>. However <that> does not refer back to anything. X out.

C) double as much as the increase of
We need a <~ing> form to modifies a whole previous sentence. X out.
D) twice as many as the increase in
E) twise as many as the increase of
<twice as...> is an appositive modifier meaning that it describes a noun immediately right or after ethe comma(in this case, "1990") However, does it make sense? No. Read again aloud.

(A) is the correct answer. <doubling...> correctly modifies the whole previous sentence.

hope this helps.

FOR 1ST
a) IS CORRECT AS SHORT AND CONCISE .ALTHOUGH THE VERB BORN IS OMMITED ,ITS ACCEPTABLE WITH AS AND THAN TO OMIT VERB IF MEANING IS NOT ALTERED

b) BORN CAN BE OMITED
C) THEY AMBIGUOUS
D) PRALLELISM ERROR IN VERB TENSE
E) SAME AS D
FOR MORE CLARIFICATION ON ELLIPTICAL CLAUSES CHECK THIS OUT
https://www.beatthegmat.com/elliptical-c ... 40219.html

FOR 2ND,

A COMMA -ING VERB FORM IS ADVERBIAL MODIFIER AND MODIFYIES THINS OTHER THAN NOUN, FURTHER IT GIVES AN ADDITIONAL INFORMATION ABOUT PREV CLASUE

HENCE a) IS CORRECT AS PER THE ABOVE RULE

https://www.beatthegmat.com/ing-modifier ... 38943.html

JUST CHECK THE ABOVE LINK ITS THE BIBLE OF btg DISCUSSION,