For decades Surgeon Generals have warned that smoking

Can you please post the Official explanation (OE) for C and D.

C and D are close.

In C: increasing the risks of low birth weight - an explainatory clause modifies the noun, pregnancy.

In D: This seems to be a passive construction. I have doubt on part with BY. The construction by increasing the risks of low birth weight says that smoking by increasing the risks of low birth weight can increase...and can complicate...:

For decades Surgeon Generals have warned that smoking [increases the risk of lung diseases and can complicate pregnancy] by increasing the risks of low birth weight.

Uhh, I just noticed that I did a big mistake. Sorry guys for that. I posted several questions from Kaplan the other day, and I must have confused it. I'll have to review the other questions as well. Sorry for the inconveniences.

Official Answer follows:

The original sentence has an unnecessary and before can complicate, but its larger problem is that the final item, increases the risk of low birth weight, should be subordinated to the previous, can complicate pregnancy, since the two problems are related. Choices (C), (D), and (E) all subordinate the final section by using a participle (increasing). (C) loses out in comparison to (D) and (E) since increasing is less clear than by increasing. (D) is better than (E). (E) wrongly subordinates can complicate pregnancy by replacing it with complicating pregnancy. The change makes no sense because, by subordinating this phrase to the previous one, (E) suggests that pregnancy complications are a type of lung disease. (D) it is.

So, you mean to say D is the OA or C?

The OE only says that C loses out in comparison to D and E since increasing is less clear than by increasing. I hope this is helpful.

Though I marked C, but I when I later ponder over it, D really makes sense.
You may find the attached explanation useful.

Parallel form : Smoking can increase + can complicate
E: is out
A and B : Also not parallel, if these options treats "risk of low birth rates" as one MORE disease/effect.
C is tricky but confusing
D: Correct

GMATNinjaTwo
My question is how to know exactly the main idea of a SC sentence.
I picked C b/c I thought "pregnancy leads to higher risks of low birth weight"
D is actually the answer.

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