LSAT 14 - Section 4 - Question 19

This is a necessary assumption question because the stem tells us that the argument “depends on the assumption.”

Necessary assumption questions fall under the subset of strengthen questions. All of the things we learned for strengthen, SA, and PSA question are still very important here! We’re going to be analyzing the stimulus the same way: identify the premise and conclusion, evaluate the argument, and determine what is missing. Our approach to the questions is very different. For NA question, in order for our conclusion to be true, our correct answer must be true. Without the correct answer, our argument will fall apart. This is what we’re looking for in our answer choice. Remember, we can always test our answer choices by using the negation test: if we negate the answer choice and it destroys our argument, it is the correct answer!

The first sentence is pretty straight-forward: the permits are issued in terms of pounds of each chemicals that can be dumped into the waterway per day. So, each chemical has a specified weight that can be dumped in the water each day. We’re also told how these numbers are calculated: by looking at the effects dilution in the water based on the amount of water flowing through the waterway. So there’s two variables here: the dilution and how much water passes through the waterway. This makes sense: if you have more/less water flowing through the waterway, dilution will change. Great! The next sentence is the conclusion: based on all of this information, the waterway is protected.

On the LSAT, there is a correct amount of skepticism you need to have. The argument is okay but, while it’s difficult to pinpoint, there is a gap here. A number of things could happen in order for this conclusion to not be true. For example, what if no one follows these guidelines? What if calculation of water flowing through the waterway isn’t accurate because the flow of water changes through the day? Any number of things need to be true if our conclusion, the waterway is protected, is true.

Correct Answer Choice (A) This is necessary for the argument! If, even in safe quantities, chemicals interact to form harmful ones, that would destroy our argument.

Answer Choice (B) Is rapid dispersion necessary? Maybe the flow is very slow, but the amount of chemicals allowed in the waterway is very low. This is not good.

Answer Choice (C) This is also not necessary. Let’s take the negation of this sentence: some chemicals are prohibited from being discharged into the waterway. So, what? Perhaps arsenic isn’t not allowed to be dumped - is that compatible with the argument? Yes! Prohibiting certain chemicals to be dumped in the waterway does not destroy the argument.

Answer Choice (D) This is also not necessary. The permits indicate the max amount that can be dumped into the water, which means dumping anything at the maximum amount or below is fine. Negating this sentence (they dump the full amount indicated by the permit) does not destroy the argument.

Answer Choice (E) This weakens our argument a little bit - it mentions what we’re not taking into consideration in an effort to say that the waterway may not be as protected as the argument says. This is out.