Please substantiate your answer with reasoning. It will help.
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Getting defeated is just a temporary notion, giving it up is what makes it permanent.

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such a tricky question
Now, I think D is the correct one. Both A and D look correct, but what makes D the correct one, is the part that says "Thus, tetracycline in their food probably explains..."

Will vote for D because

Question type : Assumption (Strengthen the conclusion)

conclusion : tetracycline in food explains the low incidence of typhus among ancient Nubians.

Premise 1: skeletons do show deposits of tetracyclinean antibiotic produced by a bacterium common in Nubian soil.

Premise 2: This bacterium can flourish on the dried grain used for making two staples of the Nubian diet, beer and bread.

A. The tetracycline deposits did not form after the bodies were buried.
--> one of the premise already state that "skeletons show the usual evidence of this disease", so option A cannot be true

B. The diseases other than typhus to which the ancient Nubians were exposed would not be affected by tetracycline.
--> Out of scope nothing talking about another diseases

C. Typhus is generally fatal.
--> Doesnot strengthe the conclusion.

D. Tetracycline is not rendered ineffective as an antibiotic by exposure to the process involved in making bread and beer.
--> Provide additional evidence that will strength the conclusion. If the Tetracycline which acts as antibiotic is not destroyed in the process of making bread or beer, then we can correctly conclude that
tetracycline in food probably explains the low incidence of typhus.
Correct

using ANT
Tetracycline is rendered ineffective as an antibiotic by exposure to the process involved in making bread and beer.
This clearly contradicts the conclusion, if Tetracycline doesnot act as antibiotic, then incidence of typhus would have been higher

E. Bread and beer were the only foods eaten by the ancient Nubians which could have contained tetracycline.
--> wrong because thay cannot be only source as they were made from grains and bacterium flourished on the dried grain.

ChrisLele
The conclusion of this argument is "Tetracycline in their food probably explains the low incidence of typhus among ancient Nubians" as in the last sentence. This means that author expects food to be a reason for presence of Tetracycline. I understand that presence of tetracycline can be linked a defender kind of assumption, but we also know that tetracycline has always been there in Nubian soil, making me to go away from A, and the conclusion here should defend the low incidence of typhus( and not the presence of tetracycline on skeletons).
Let me know if this sounds logical or even if there is some flaw here.

The source of this question is a CR document from a gmatclub member. It says the question is from gmatclub or MGMAT. Just hoping for this to be true now
gmat-prep-critical-reasoning-collection-106783.html#p840353
_________________

If you know what you're worth, then go out and get what you're worth. But you gotta be willing to take the hits, and not pointing fingers saying you ain't where you wanna be because of anybody! Cowards do that and You're better than that!
The path is long, but self-surrender makes it short; the way is difficult, but perfect trust makes it easy.
Fire the final bullet only when you are constantly hitting the Bull's eye, till then KEEP PRACTICING.
Failure establishes only this, that our determination to succeed was not strong enough.
Getting defeated is just a temporary notion, giving it up is what makes it permanent.

http://gmatclub.com/forum/1000-sc-notes-at-one-place-in-one-document-with-best-of-explanations-192961.html

Press +1 Kudos, if you think my post gave u a tiny tip.

I am questioning the source of this question, because BTG http://www.beatthegmat.com/nubians-t5138.html has the same question but one different answer choice.

It was a tricky question!, but a good one. I choose A at the beginning but I was wrong. Reading carefully the conclusion and the premises that support D makes more sense.
First of all this is a causal argument:
Conc: Tetra in food --> probably explains low incidence of Typhus
P1: Skeletons of N show deposits of Tetra (antibiotic produced by a bacteria in N's soil)
P2: Bacteria can flourish on the dried grain used for making N diet: beer and bread.
P3: N inhabited an area with typhus, yet surprisingly few of their skeletons show evidence of this disease

Pre-thinking:
For a causal argument to be true:
-No other causes provoke the effect
-The effect doesn't cause the "cause"
-The cause happened before the effect.

Option A) The tetracycline deposits did not form after the bodies were buried. Incorrect
This option is apparently aligned with the latter one (sequence of events); however, option A doesn't talk about the real cause mentioned in the conclusion and supported by premises: Tetra in food. We know (from premises) that Tetra was indeed present in N's diet. So this is a real fact. Option a provides irrelevant information because even if Tetra deposits formed after the bodies were buried, the few evidence of the disease in the skeletons could or could have not happened because of Tetra in food.

Option D) Tetracycline is not rendered ineffective as an antibiotic by exposure to the process involved in making bread and beer. Correct
If the Tetra in food had loose its properties as an antibiotic after it was processed to beer and bread, It would have been ineffective to prevent the disease. So the few evidence of the disease in the skeletons would have been attributed to other causes.

The ancient Nubians inhabited an area in which typhus occurred, yet surprisingly few of their
skeletons show the usual evidence of this disease. The skeletons do show deposits of
tetracycline, an antibiotic produced by a bacterium common in Nubian soil. This bacterium
can flourish on the dried grain used for making two staples of the Nubian diet, beer and
bread. Thus, tetracycline in their food probably explains the low incidence of typhus among
ancient Nubians.

Which of the following is an assumption on which the argument relies?
A. The tetracycline deposits did not form after the bodies were buried.
B. The diseases other than typhus to which the ancient Nubians were exposed would not be
affected by tetracycline.
C. Typhus is generally fatal.
D. Nubian grain became contaminated with tetracycline-producing bacteria prior to being
harvested.
E. Bread and beer were the only foods eaten by the ancient Nubians which could have contained
tetracycline.
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Regards ,

There is one more point, which makes A weak as an assumption. If tetracycline deposited from the soil into the skeletons of these bodies, it still does not explain why only "few of their skeletons showed the usual evidence of typhus. After all, the ancient nubians inhabited in the area where typhus occurred, making them more susceptible to the disease. Most of them would have incurred the disease when they were alive. Typhus would invade their bodies when they were alive.
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Cheers,
Varsha Madhav
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