We haven't seen our neighbors for over a week.They -- on a t

We haven't seen our neighbors for over a week.They __ on a trip abroad.
a:could go
b:must go
c:may have gone
d:shoud have gone
答案书后面写着C
很奇怪哦.如果说may是猜测的话.既然用了may而不是might为啥后面却跟者现在完成时呢?如果是may go还可以理解-.- 这算什么时态或者句法呢?前面是普通时后面却用完成时.