The object which Shawn saw on his way home last night could had been a UFO but, because he was traveling close to the airport, he wasn't sure.

a) could had been a UFO but, because he was traveling
b) might have been a UFO but, therefore it wasn't likely since he traveled
c) may have been a UFO but, since he was traveling
d) might possibly had been a UFO but,
e) could have been a UFO but likely not since it could be

OA is C
What makes option C unique? i need an Expert to help me explain this! Thanks