So far no life has been found outside the earth, but while some scientists are actively searching for living things in outer space, others firmly believe there ____ . 

To my surprise, my good friend _________ stay up to study to prepare for the next important exams organized by ten famous high schools.

It has been announced that candidates       remain in their seats until all the papers have been collected.

．—What’s the matter with the man hanging his head there? —Well, if you _______ know, he was caught stealing my bike.

 “Look at these tracks. It _______ be a wolf.” “Don’t be so sure. I think it _______ be a fox.”