#13 - Ethicist: Studies have documented the capacity of

[Adam Tyson]

The negation of the correct answer should not match the prephrased assumption; it should ruin the argument. Your negation actually helps the argument, the conclusion of which is that administering placebos is ethically questionable. We don't want an answer that says placebos are justified; we want one that says they may be a problem!

To simplify, the argument is "placebos might be bad, because sometime the doctor has a bad motive."

The assumption is that the motive of the doctor matters. What if it doesn't matter? Then the argument falls apart. The negation of B ruins the argument, proving that B was indeed necessary.