Both (A) and (B) are incorrect. (D) is the correct answer.
The passage tells us that some people believe mining the Moon for helium-3 to be used as fuel in fusion reactors can solve the world's energy problems. The passage then says that "this is nonsense." Notice that this indicates a shift in the flow of the passage and is the conclusion/main point of the argument. Further support is then offered for this conclusion- that, even if mining the Moon for helium-3 is possible, the technology needed to build fusion reactors that use such fuel is 50+ years away and the energy problems need to be solved before then.
We are asked to identify the main point of the argument.
(D) "mining the Moon for helium-3 is not a possible solution to the world's energy problems" This correctly restates what we identified as the conclusion and main point of the passage. Overall, the argument is saying the proposed solution of mining the Moon for helium-3 is nonsensical because it cannot solve the world's energy problems.
(A) "mining the Moon for helium-3 is currently not feasible" The argument does indicate this, but it is not the main point of the argument. Moreover, we should notice the difference between mining being currently impossible (which is what this answer choice says) and it being possible in the future yet ultimately still unsuccessful at solving the world's energy problems (which is the main point of the argument).
(B) "fusion reactors that are now being planned are not designed to use helium-3 as fuel" As with answer choice (A), answer choice (B) misses the larger conclusion- that mining the moon for helium-3 cannot solve the world's energy problems. Additionally, (B) contradicts information in the passage that implies reactors are 50+ years out that are planned to use helium-3 as fuel.
Does that make sense? Please let us know if you have any other questions!
