Choice B is designed to be a specific type of distraction, let me explain and you can tell me what you think. The way that I would describe the reason that B is not correct is this: B indicates that possible evidence for the presence of the ivory-billed is not there (i.e. the peculiar sound of the tapping). Yet it is not stated that this sound will always be heard. So this is what I would call sufficient evidence, but not necessary evidence. If you have the sound, then you likely have the woodpecker (it is sufficient). But the absence of the sound, does not mean the absence of the woodpecker (not neccessary).
On the other hand, choice D is directly weakens the only evidence that they have for the ivory-billed, the large white wing patches. It is true that since the patches are present for both types of birds it is still possible that this is the ivory-billed, but since that bird has not been seen for decades it is very damaging to have the only evidence in favor of the bird being an ivory-billed turn on not to be a true indication of that bird. (As to the video versus observations, I think that seeing the wing patches on the video can be considered an "observation.")
On the other hand, choice D is directly weakens the only evidence that they have for the ivory-billed, the large white wing patches. It is true that since the patches are present for both types of birds it is still possible that this is the ivory-billed, but since that bird has not been seen for decades it is very damaging to have the only evidence in favor of the bird being an ivory-billed turn on not to be a true indication of that bird. (As to the video versus observations, I think that seeing the wing patches on the video can be considered an "observation.")
