Not necessary, because the author’s arguing the meteor that made the crater didn’t cause the extinction. So if (A) were negated — if the impact was NOT of enough size to cause the mass extinction — that doesn’t undermine the author’s reasoning.

Necessary, because if it were not true — if the recovered rocks recrystallized MORE than shortly after they melted — then that could explain why the polarity of the recrystallized rocks is different from the polarity at time of extinction. The meteor could have hit, caused extinction, and then the rocks recrystallized later.

Necessary, because if it were not true — if some OTHER event DID cause the rocks to melt after the impact formed the crater — then that could explain why the polarity of the recrystallized rocks is different from the polarity at time of extinction. The meteor could have hit, caused extinction, and then after the rocks recrystallized at that time, they could have melted and recrystallized again later, with a different polarity.

Necessary, because if it were not true — if the rocks did NOT melt as a result of the impact that formed the crater — then the rocks could have recrystallized after the impact and extinction, which explains why they have a different polarity from the polarity at time of extinction.

Necessary, because if it were not true — if the mass extinction would NOT have occurred soon after the impact — then that could explain why the recrystallized rocks have a different polarity from the polarity at time of extinction. The meteor could have hit, the rocks recrystallized, and later the mass extinction resulting from the meteor occurred.