PT113 · S4 · Q18 A plausible explanation of the

This just tells us that an Alternate Explanation exists, but it doesn't give us any reason to prefer that explanation over this one. There's a difference between an answer choice providing the grounds for an alternate explanation (i.e. "There is strong evidence of a microbial infection that is known to kill present day reptiles around the same time as the dinosaurs' disappearance") and saying that someone else has an alternate explanation (i.e. "Eddie thinks that a microbial infection killed them). To put that another way ... being given a fact that could lead us to an alternate explanation is more powerful than just being told that someone has an opinion that involves an alternate explanation.

This means that skeletons do not provide evidence in favor of the theory, but that doesn't mean it's evidence against the theory. An absence of evidence is not evidence that something is wrong. (that's the famous Unproven vs. Proven False flaw in a nutshell)

This is not as good as (B) because these other animals might be very different from dinosaurs. If hermit crabs in Hawaii didn't die off, that's not necessarily counterevidence. Dinosaurs are 1,000 times bigger. They don't live in Hawaii. They're reptiles, not crustaceans. (B) is better because it's saying that animals that had a similar habitat and similar physiology were not killed off.

The fact that we don't fully understand X doesn't suggest that X isn't true. Much like (C), this answer is saying "we lack certain knowledge / evidence", but that doesn't mean that there aren't consequences for vegetation and animals when a comet collides with Earth. Also, to say something isn't fully understood is a very weak knock on it. We might feel like we have a 95% understanding of genetics. We don't fully understand it, but we understand it a lot!