You need to ask yourself why the Engineer is making her argument. Her point is not that we cannot mine the Moon for helium-3, but that those who believe mining the Moon for helium-3 could solve the world's energy problems are wrong.
The Engineer states that even if it were possible to mine the Moon for helium-3 (implying that it is currently not), the technology needed to build viable fusion reactors that could use such fuel is at least 50 years away; and if the world's energy problems are not solved before then, it will be too late to solve those problems. So her point is not that it's not currently possible, but that this is just not a possible solution because even if it were possible to solve the world's energy problems, it would take 50 years, which is essentially too late.
That is the difference between answer choice (A) and (D). (A) states that mining the Moon for helium-3 is currently not feasible, which, though according to the stimulus is true, is not our main point.
Answer choice (D) states that mining for helium-3 on the Moon is simply not a possible solution to the world's energy problems, which is our main point as explained above.
Hope that helped! Please let us know if you have any more questions.
Think GreenJanuary 22, 2014
Thank you! I got it now. A is just like a premise or subsidiary conclusion which supports D. In this sense, D is the right one.
stormbeelerAugust 29, 2015
Is answer choice E simply a premise strengthening the claim that the mining is nonsense? Thanks!
