Critical Reasoning

Researchers studying the spread of the Black Plague in sixteenth-century England claim that certain people survived the epidemic because they carried a genetic mutation, known as Delta-32, that is known to prevent the bacteria that causes the Plague from overtaking the immune system. To support this hypothesis, the researchers tested the direct descendants of the residents of an English town where an unusually large proportion of people survived the Plague. More than half of these descendants tested positive for the mutation Delta-32, a figure nearly three times higher than that found in other locations.

The researchers' hypothesis is based on which of the following assumptions?


a. Delta-32 does not prevent a carrier from contracting any disease other than the Plague.


b. The Plague is not similar to other diseases caused by bacteria.


c. Delta-32 did not exist in its current form until the sixteenth century.


d. No one who tested positive for Delta-32 has ever contracted a disease caused by bacteria.


e. The Plague does not cause genetic mutations such as Delta-32.

Can anyone explain why option d is wrong?

I think it should be option 'b'
Option D is incorrect because the passage specifically talks of plague causing bacteria and does not talk of all diseases caused by bacteria.
Also option b is correct because if Plague is similar to other diseases caused by bacteria then delta 32 should prevent the people from all bacterial diseases.

Consider what an extreme sentiment 'd' expresses:

d. No one who tested positive for Delta-32 has ever contracted a disease caused by bacteria.


Imagine that you have this special gene. The gene allowed your ancestors to survive the plague, and subsequently, it was passed down to you. Would you assume, then, that because you have this gene you'll never contract any disease caused by bacteria? Of course not. You can still get sick. You're just less likely to die of the Plague, which, to be fair, is still a pretty nice perk.

What is the answer to this question and more importantly why?

Here we have a classic causality argument. In essence, having Delta-32 allows people to survive the plague.

There are two common potential problems with a causality argument. In this case, maybe something else, other than this gene, allowed these people to survive the Plague. Second, maybe cause and effect are reversed, and somehow contracting and surviving the Plague led to people getting this gene.

Watch what happens to answer choice E when we negate it: The Plague does cause genetic mutations such as Delta-32.

Well, if this is true, then survivors likely didn't survive the Plague because they had this special gene, but rather they have this special gene because they survived the Plague. There's no reason to believe that the gene confers any benefit at all. So the negation of E utterly destroys the argument by reversing cause and effect. Anytime you have an assumption question, and the negation of an answer destroys the argument, you know that answer must be correct.