sk05
I didn't understand the question type. Isn't the question asking us to weaken the support and not the conclusion?
In my opinion, the support is that there is less frequency of blood poisoning when M and N are given as compared to when only M is given.
Yes, the support is "when mice are given both morphine and the new drug naltrexone, blood poisoning is much less frequent."
Regarding the question type, this question is a Weaken the Argument question.
In general, in a Weaken question, the correct answer weakens the case for the conclusion. It does so by showing that, even though the facts presented are true, the conclusion may not be correct.
So, the correct answer does not indicate that the supporting premise may not be true. Rather, it may weaken the support provided by the premise.
For instance, in this case, the reasoning is that, since blood poisoning is less frequent when naltrexone is given, naltrexone must be toxic to the bacteria that would normally cause blood poisoning.
The correct answer shows that, even though blood poisoning is less frequent when naltrexone is given, naltrexone may not be toxic to bacteria since something else may be going on: it may be that naltrexone instead causes the bacteria not to migrate into the bloodstream.
So, the correct answer provides additional information that indicates that the conclusion may not follow from the premise. In other words, the correct answer weakens the support provides by the premise.
Thus, the correct answer is a classic Weaken correct answer.