The room is in a terrible mess; it ________have been cleaned.
The research lacks evidence, and therefore, its conclusions are doubtful.
We live in an age more information is available with greater ease than ever before.
I think Tom, as the head of a big department, should either study regularly or______ his job.
Youras a student will be excellent if you develop a habit of reflecting on how you learn.
all the animals I've ever had, these two dogs arc the most sensitive to the spoken word.