What Conclusion Did the U

What conclusion did the U.S.Air Force draw in their Blue Book report?

A) Evidence from alien autopsies confirm that UFOs exist and merit further study.
B) Since there is no evidence that UFOs exist, we can safely conclude that they do not exist.
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) All UFO sightings can be explained as hoaxes or natural phenomenon such as ball lightening.