This has to be the worst "correct" answer choice I have ever seen. I feel sorry for the 7Sage writers and tutors having to pretend that the "This may be impractical..." sentence distances the author from the argument, when it actually obviously does the complete opposite.

The only way Answer Choice A would ever be correct is if the second paragraph ended with the following parenthetical: "(note: I am not necessarily personally endorsing this entire way of reasoning that I have just laid out in detail)".

I think the key word isn't just "if", it's also "could feed".

If that sentence were to read "So if steel-manufacturing plants fed the heat they produce into thermophotovoltaic generators...", then B would be a necessary assumption. In order for the plants to engage in this process in the first place, it would be necessary for the technology to actually exist for this process to happen.

I think the reason I got this wrong and chose B (even though I knew C "also" made sense as an answer) was because I didn't interpret "if...could feed" as meaning "if it was possible", creating a "hypothetical world" where it is possible. I misread it instead as meaning something like "if they bothered to".