The conclusion: the doctors who recommended the surgery were doing so for selfish reasons (a chance to hone their skills/a chance to make $)
The premise: 25% of the patients undergoing the operation did not benefit.
The argument is asserting that the doctors, in some cases, knew that a patient was a bad candidate for the surgery, but recommended the procedure anyway. (Imagine a doctor evaluating a patient, thinking to herself, "yikes, there's no way this procedure will work," and then doing it anyway for the money.)
If we want to undermine that claim, we want to show that the doctors were not consciously thinking that some of these patients would make poor candidates for the procedure.
D is irrelevant. We're assessing the knowledge/motivation of the doctors, not of the patients.
E: If it were impossible to tell the difference between the patients who would benefit and those who wouldn't, then doctors couldn't possibly have been thinking to themselves that some of the patients were poor candidates for the procedure, as they looked just like the ones that benefited. So the scenario above, where the doctor is thinking to herself "bad candidate, but let's make some money!" is no longer plausible. E is the answer.
Moreover, The conclusion here is the last sentence: the doctors were more interested in practicing their skills and making money than in helping the 1 in 4 patients who did not benefit from the surgery. This assumes without providing justification that there's no other reason that doctors would perform the surgery on people who did not benefit from it. However, E weakens the argument by explaining that there was no way to know who would or wouldn't benefit from the surgery; prior to the surgery, all of the patients seemed the same, so until they actually operated and saw the outcome, the doctors had no idea which patients would see good results and which ones wouldn't. Therefore, they weren't necessarily trying to practice their skills and make money on the 25% that didn't see good results. They might have really been trying to help everyone, and 25% of the time, they just failed for some reason.
Just wanted to note that the question says "undermines."
"undermines" = weakens argument
"underlies" = basis for argument, supports
Good to watch out for those two very close-looking words!
E is the best answer choice here.
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Thanks & Regards,
Anaira Mitch