—She looks very happy. She ______ have passed the exam.—I guess so. It’s not difficult after all. 

Now then, children, Its time you ________.

—How’s your tour around the North Lake? Is it beautiful?—It          be, but it is now heavily polluted.

He ________ the exam, but he was too careless.

—This wall of air is an effective answer to the problem  and it ____ work.  —But it is a question to put such a wall of air around the statue.