Logical reasoning PrepTest 148 · Section 1 · Question 25

Incorrect. The argument relies on the colder (and deeper) ocean water trapping the carbon dioxide for centuries—the reason to use that cold water is for that reason, not because it dissolves more thoroughly.

Incorrect. The stimulus is about pumping carbon dioxide into the deeper water, not the surface water, so this answer is out of scope. The argument itself also even concedes that the carbon will eventually evaporate when it reaches the surface.

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Strengthen with Necessary Premise

Stimulus Summary:
Cold, deep, dense water stays submerged for centuries, so any carbon dissolved in that water will stay trapped for centuries, and thus we should pump carbon deep into the oceans.

Answer Anticipation:
This argument features an intermediate conclusion and a main point, so we should look for assumptions at each step.

We're going to start with the main point—it concludes that something should be done because it'd work to get carbon dioxide out of the atmosphere for a few centuries. The LSAT is going to assume that we should fight climate change, but this conclusion takes a solution that would work on one metric and concludes that it should be done. That assumes that the overall balance is positive—i.e., it assumes there are no negatives that mean we shouldn't try this. If pumping that much carbon into the deep ocean water kills all the fish, that might be a problem, so the argument is assuming that this wouldn't have any catastrophic effects that are important enough to recommend against doing this.

For the intermediate conclusion, is there an assumption between the cold water not mixing with surface water for centuries and the carbon staying trapped? Sure! The carbon might not stay dissolved in the water for long, or it might mix faster than the water does. The argument is assuming that if the water stays submerged, then so will the dissolved carbon, which isn't necessarily true.

Either assumption could lead to a correct answer, so let's dive into the answers and look for both!

Answer Explanation:
If the carbon that is pumped into the water will make its way back to the surface "long" before the water itself does, then it's questionable whether the carbon will stay trapped for the "centuries" that the Oceanographer claims. Since the argument is based on this colder, denser water not mixing with the surface water, the argument requires assuming the carbon will stay with it, making this answer necessary.

Key Takeaway:
When there's an intermediate conclusion in a Strengthen with Necessary Premise question, be sure to anticipate any assumptions made at each step of the argument. There's nothing worse than eliminating all five answers because they don't match your anticipation—it can be hard to know where to go after that!

Incorrect. The argument requires that the carbon get trapped by the cooler, denser water, not that the density be the reason for it. If the temperature is what allows it to trap the carbon, the argument works just as well.

Incorrect. The Oceanographer does claim this solution will work for centuries, but the argument doesn't require that it work for centuries. If the plan just worked to buy time to come up with other solutions, it's still possible that we should do it.