I get why C is right, but why is B wrong?

@Deke, Happy to help. Let's look at (B). (B) says, "Using current technology, it would be possible for steel-manufacturing plants to feed the heat they produce into thermophotovoltaic generators in such a way that those generators could convert at least some of that heat into electricity." (B)'s negation says, "Using current technology, it would be impossible for steel-manufacturing plants to feed the heat they produce into thermophotovoltaic generators in such a way that those generators could convert at least some of that heat into electricity." The problem with (B) is that the author merely made the claim that "if steel-manufacturing plants could feed the heat...", and this does not imply "current technology." Maybe current technology can't do this. If (B) is negated, the argument doesn't fall apart, so this is why (B) isn't a necessary premise. Hope this helps. Let us know if you have any other questions!

Thanks for the explanation. That was a tricky one.