A. Since there is no evidence that UFOs exist, we can safely conclude that they do not exist.
B. All UFO sightings can be explained as hoaxes or natural phenomenon such as ball lightening.
C. Most UFO sightings can be explained without accepting the existence of extraterrestrial beings, but a small number of these sightings have yet to be explained and deserve continuing study.
D. Evidence from alien autopsies confirm that UFOs exist and merit further study.