I hear your frustration, lanereuden! The key to answering this question is in this phrase from the stimulus:
In the limited areas of the world where such underground hot water and steam can currently be reached by drilling
The author is telling us that geothermal plants are only workable in a few places, because those are the only places where we can drill down to where the hot water and steam are. If in the near future we could drill farther down, that would likely mean that we could reach some hot water and steam that is not
currently reachable by drilling. It's the reachability that is the limiting factor, not the cost!
Answer E doesn't help us any, because it does nothing to increase the general availability of hot water and steam to feed a geothermal power plant. Costs can come down to zero, but if you can't get the fuel because your drills won't reach it, what good does that do you?
Remember not to fight with the answer choices on the LSAT! That's a fight that you can never win, and which produces no useful outcomes but only leads to frustration and missed points. Instead, look for what the authors were thinking, and try to understand why THEY think an answer is good, or why another is bad. Once you can get into their heads, and out of your own, you will find the test much easier to understand and attack.
Good luck, keep up your spirits and the good work!
Adam M. Tyson
PowerScore LSAT, GRE, ACT and SAT Instructor
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https://twitter.com/LSATadam