(A) establishes that in order for there to be a consistent causal link between the impacts and mass extinctions, then ALL major meteor impacts would be followed by such extinctions. We know from one of our premises that many major meteor impacts were NOT followed by such extinctions. This triggers the contrapositive of (A), which would establish that there is NO consistent causal link.

(B) establishes that in order for there to be a consistent causal link, it’s required that many mass extinctions have followed major meteor impacts. But we don’t know that this requirement hasn’t been met. Although we do know that many mass extinctions have not followed major meteor impacts, it’s still possible that many such extinctions have followed such impacts, too.

(C) doesn’t establish what is required for a “consistent causal link” between extinctions and meteor impacts, so it can’t make the argument valid.

(D) establishes what would be true IF there is no consistent causal link. But we’re trying to reach the conclusion that there is no consistent causal link. We don’t want “no consistent causal link” to appear in the “IF” part of an “If, then” answer. We want it to appear in the “then” part.

(E) tells us about one circumstance under which there can be a consistent causal link. But we’re trying to prove that there’s NO consistent causal link.