A study of expectant mothers suggests that thin women can si

Conclusion presents a causality that
By taking zinc supplements during pregnancy ---- >>thin women can significantly increase their chances of bearing bigger babies .

Only Option A defends this causality by eliminating alternate cause for stated effect.

A) Zinc found in natural sources, such as seafood, nuts, and milk, is not as effective as supplements in increasing a pregnant woman's chances of bearing larger babies.

B) The first 18 weeks of pregnancy did not play a significant role in the results of the study.
First 18 weeks play significant role in babies growth, but these first 18 weeks are same for both 300 and 250 groups of women. Only known change that is addition of supplement is introduced in 19th week. Thus, even if first 18 weeks play significant role, we can not see that it is filling any gap between premise and conclusion. we can not assume that women of both groups were on some different special diet or under different special condition and such difference can help thin women to carry bigger babies... such assumption is out of scope of this argument.

B is not the biggest assumption that the passage makes, but the argument doesn't rely on any other of the other options. Go with the best answer.

Premise: Three hundred women who took 25-milligram zinc supplements starting with the 19th week of pregnancy were compared to 250 women who took placebos.
Premise: All of the women also took a multivitamin-mineral supplement containing no zinc.
Premise: Babies born to the zinc group weighed five ounces more on average, with the greatest gains among thin women; for example, women in the zinc group who were 5' 4" tall and weighed under 110 pounds had babies one pound heavier, on average, than those born to similar-sized women in the placebo group.

CONCLUSION: A study of expectant mothers suggests that thin women can significantly increase their chances of bearing bigger babies by taking zinc supplements during pregnancy.

(The first line of the argument is the conclusion here because it tells about the result of the study conducted.)
WOMEN…………..INCREASE THEIR CHANCES OF BEARING BIGGER BABIES…………..BY TAKING ZINC SUPPLEMENTS……………..AND THAT’S IT (NOTHING ELSE). That means there is no other reason for this result….ONLY zinc supplements.
We have to look for a choice that clearly removes a possibility of any other factor contributing to this result.

Let’s check the options:

A) Zinc found in natural sources, such as seafood, nuts, and milk, is not as effective as supplements in increasing a pregnant woman's chances of bearing larger babies.
OUT OF SCOPE. We are not concerned whether zinc from OTHER SOURCE is beneficial or not.

B) The first 18 weeks of pregnancy did not play a significant role in the results of the study.
This is the correct choice.
Let’s negate it: The first 18 weeks of pregnancy DID PLAY A SIGNIFICANT ROLE IN THE RESULTS. Then Zinc is not the only reason. This shatters the conclusion.


C) Zinc supplements have little or no effect on the size of babies born to taller, heavier mothers.
OUT OF SCOPE

D) Thin women who take more than 25 milligrams of zinc daily will give birth to even larger infants.
OUT OF SCOPE

E) Zinc supplements have a greater effect on the size of babies born to thin women because these women have less zinc stored in their bodies.
OUT OF SCOPE

VeritasKarishma GMATNinja nightblade354, could you please see if my reasoning to negate E is correct?
E is wrong because it gives the reason which may or may not be possible. there might be some other reason as well due to which the size of babies increased