Not necessary, because even if there’s no change in the age category of people struck by degenerative brain disorders, over time, as people live longer and longer, we might expect people with brain disorders to make up a larger proportion of people. The author’s reasoning is not that brain disorders strike younger people, but that more and more people will survive long enough to eventually have a degenerative brain disorder.

Not necessary, because whatever is true about the current frequency of transplants or need for transplants is not relevant. The argument concerns what will happen in the future, as transplantations become more common and allows more and more people to live longer.

Necessary, because if this were not true — if there are NO degenerative brain disorders that will not be curable without brain transplants (in other words, if every deg. brain disorder is curable without brain transplant) — then the fact we can’t do brain transplants does not prove that more and more people will have degenerative brain disorders in the future. Even if we can’t do transplants, we might still be able to cure degenerative brain disorders. The author must assume this isn’t possible.

Not necessary, because whatever is true about the current state of degenerative brain disorders and deaths is irrelevant. The argument concerns what will happen in the future, as transplantations become more common and allows more and more people to live longer.

Not necessary, because the amount spent on research is irrelevant. Nothing in the argument cited to or relied on comparative amounts of research spending.