Please substantiate your answer with reasoning. It will help.

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

This is a valid GMAT question. However, if the gmat gives such a question, it will not include both choices A and D in the same question, because both are correct.

The same question has been asked, but in place of choice (A), a different choice has been stated. See here:
the-ancient-nubians-inhabited-an-area-in-which-typhus-occurs-44937.html

Again, in this question, both choice A and D are correct.

Facts: few of the skeletons of Nubians show the usual evidence of the disease typhus even though they inhabited an area in which typhus occurred,. But their skeletons 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.

Missing Information:There is no alternate explanation to what is shown in bold or it is not falsified, such as Tetracycline being rendered ineffective as an antibiotic by exposure to the process involved in making bread and beer.

Conclusion:Thus, tetracycline in their food probably explains the low incidence of typhus among ancient Nubians.

Choice D is the best match for the missing information.

Choice A seems very close. To eliminate one of the two, we have to focus on the reason for the conclusion. It is shown in bold. It means that the author believes that tetracycline was their in their food and this would eliminate A because choice A is more a premise. So actually attacking author's argument is showing tetracycline in the food is not responsible for the low incidence of the disease.
So the assumption of D saying that the tetracycline in the food is effective, is required.
_________________

Srinivasan Vaidyaraman
Sravna Holistic Solutions
http://www.sravnatestprep.com

Holistic and Systematic Approach

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.
_________________

Cheers,
Varsha Madhav
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Kudos please if my post helped you...!!

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) Tetracycline is not rendered ineffective as an antibiotic by exposure to the process involved in making bread and beer.

(E) Bread and beer were the only foods eaten by the ancient Nubians which could have contained tetracycline.

negating the main-point:
1. TT is not the reason for low traces of TY
given that a) TY is common disease b) TT found in staple food and is Anti-biotic to TY

pre-think:
1. what if Ty is not affected by TT because of something
2. what if Ty is not properly captured due to poor tech
3. what if some other pathogen has reduced the Ty after death

look at the options:
(A) The tetracycline deposits did not form after the bodies were buried.
even if TT came after burial, it could react to Ty and decrease it. hence loose end. incorrect

(B) The diseases other than typhus to which the ancient Nubians were exposed would not be affected by tetracycline.
discussion about other dieseases is irrelevant.hence incorrect

(C) Typhus is generally fatal.
it doesnot provide any clue !!.

(D) Tetracycline is not rendered ineffective as an antibiotic by exposure to the process involved in making bread and beer.
if TT is ineffective, it cannot kill Ty. hence it cannot be the reason of low traces of Ty. hence correct

(E) Bread and beer were the only foods eaten by the ancient Nubians which could have contained tetracycline.
even if bread and beer were not the only food source of TT, it doesnot imply anything at all!.