Sentence Correction

Found this question on GMAT800 Kaplan book but I am not convinced with the answer. Can somebody please help me?

Q. For decades, Surgeons General have warned that smoking increases the risk of lung diseases, including lung cancer and emphysema, and can complicate pregnancy, and increases the risk of low birth weight.

A. Same
B. can complicate pregnancy, and increases the low birth weight risks
C. can complicate pregnancy, increasing the risks of low birth weight
D. can complicate pregnancy by increasing the risks of low birth weight
E. complicating pregnancy by increasing the risks of low birth weight

I think the answer should be C as it will is most parallel to what comes before in the sentence, which is, increases the risk of lung diseases, including.... But the book cites D as the correct answer which I don't accept because the causal effect of by is not warranted. Citing the increasing risks as a complication in pregnancy looks more logical to me as it parallels what comes before - lung diseases of which lung cancer is an example.

Can someone shed more light on this?

C. Smoking can complicate pregnancy, increasing the risks of low birth weight.
D. Smoking can complicate pregnancy by increasing the risks of low birth weight.

Both of these answer choices are viable.
Ignore this SC.

I agree this question sucks, because I don't think there is a fair, "GMAT-like", discernible grammatical or stylistic difference between the choices.

But, if we're gonna split hairs, "by increasing" is more precise than "increasing" since the word "by" gives us the meaning "the manner in which this is done." So, I believe they are making the case that since (D) gives a more SPECIFIC meaning, it is MORE clear than C.

But again, this question, in my humble opinion, is not similar to anything the GMAT would actually test. Ignore it. This is why I don't recommend Kaplan materials to my students.