break down

C does not weaken the argument since it is talking about fossils in a completely different kind of rock. The argument is that since the fossils are in the third kind, they were likely formed by a sandstorm. The existence of other fossils in a different kind of rock is not relevant to that argument. D doesn't weaken because these fossils are 65 million years old. Who cares about what happened a mere 5m years ago? E does weaken the argument by offering a possible alternative explanation for the facts that the author thinks show it was a sand slide.