Happy to help. The stimulus tells us that a group of rats was given a high-salt diet. After they received the diet, the researchers discovered that 25% of the rats had normal blood pressure, but 75% of the rats had high or really high blood pressure. The argument's conclusion is that high-salt diets are linked to high blood pressure in rats.
We're tasked with picking an answer choice that identifies a question that will allow us to evaluate the quality of this argument. Given what we know so far, we still need to have more information about the demographics of the rats in the study. If the rats were all ones with normal blood pressure to begin with, then sure, the studies results do suggest that high-salt diets are linked to high blood pressure. However, what if all of the rats in the study already had high blood pressure before the study? Then the argument's conclusion wouldn't follow, as 25% of the rats at the end of the study had normal blood pressure.
Now that we know what we're looking for, let's take a look at the answer choices.
(A) says, "How much more salt than is contained in a rat's normal diet was there in the high-salt diet?"
The problem with (A) is that this isn't an important question, as we already know that the amount of salt given to the rats met the parameters for what's considered a high-salt diet. The effects of the high-salt diet are we care about. We don't care about what exactly makes for a high-salt diet.
(B) says, "Did the high blood pressure have any adverse health effects on those rats that developed it?"
(B) doesn't matter, as we are concerned with whether or not the high-salt diets caused the high blood pressure in the rats. We aren't concerned with whether or not the rats' high blood pressure caused other health effects.
(C) says, "What percentage of naturally occurring rat colonies feed on high-salt diets?"
We only are concerned with the rats in the study. Answering (C) wouldn't tell us anything about whether or not high-salt diets cause high blood pressure in rats. It's irrelevant.
(D) says, "How many rats in the colony studied had abnormally high blood pressure before the study began?"
As we mentioned above, we want to answer the question of what the blood pressures of the rats in the study were like before they received high-salt diets. This question would give us that answer, and it would help us to determine whether or not the argument's conclusion makes sense. This is our answer choice.
(E) says, "Have other species of rodents been used in experiments of the same kind?"
If we had the answer to (E), we still wouldn't be able to tell whether or not the argument's conclusion makes sense. We need information about the rats in the study, not other rodents.
Does this make sense? Let us know if you have any more questions!
