It doesn't seem very ------ that professor Smith has left fo

(C) likely
likely形容词,表示可能的.
seem是系动词,后面要跟形容词.
it's likely that从句/it seems likely that,看起来----事情是有可能的
这句话的意思是,Smith教授已经离开去北京给美国人上课了,这件事的可能性不是很高.