新概念英语上的一道题

新概念英语上的一道题
I still don't believe in ghosts even after my experience at the Rose Inn .(B) I have never seen one.
A.because B.at least C.at most D.though
“A.D是连词,显然应该排除.”
为什么呢?我觉得这个地方就是应该填连词啊,后面是一句完整的话啊