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The argument is structured as follows:
Premise—Seawater irrigation is possible.
Premise—The greatest expense in irrigated agriculture is in pumping the water.
Premise—Pumping costs increase with the distance the water is pumped.
Conclusion—Seawater irrigation near oceans would be cheaper than most other irrigated agriculture.
Like many Method—AP stimuli, the argument does not resort to any premise or conclusion indicators to point you in the right direction: your job is to understand the structural relationship between the claims. Which claim supports (or explains) the other? Well, seawater irrigation near oceans would be cheaper because pumping the water is so expensive. The latter claim provides support for, or explains why we should believe, the former claim. Consequently, the claim referenced in the stem is simply a premise for the argument’s only conclusion—a prephrase that immediately reveals answer choice (C) to be correct.
Answer choice (A): This answer choice can be immediately eliminated, because none of the claims made in the stimulus have been disproven.
Answer choice (B): The only hypothesis presented here can be found in the first sentence—it is possible to grow crops that can thrive when irrigated with seawater. The referenced claim is not a hypothesis, let alone one that undermines the argument’s conclusion.
Answer choice (C): This is the correct answer choice, because the referenced claim supports the observation made in the second sentence of the stimulus.
Answer choice (D): The cited claim is not a conclusion, because it provides support for another claim in the stimulus.
Answer choice (E): This answer choice may be attractive, as it describes the function of an intermediate, or subsidiary, conclusion. The referenced claim is indeed not the argument’s conclusion; however, it is not a claim for which the argument provides evidence. We are never told why the greatest expense in irrigated agriculture is in pumping the water. Is the machinery expensive? Does it cost a lot to operate? We don’t know. All we know is that it’s expensive as a matter of fact.