It's four years since Mike ---- to Tibet.A.has been B.is com

句型：
It is +一段时间+since...意思是“自.发生以来已经一段时间了”.后边用一般过去时
如：
it is three years since he died.(自)他去世以来已经三年了.
而你的A属于另外一种
即：since 引导的从句,主句用现在完成时,从句用一般过去时!从句就是since后边的句子.
如：
He has studied very hard since he came to our school.
自从他来到我们学校,他学习就非常努力.
注意这个和：He has been a teacher since 1999.仔细观察哦,这个是简单句