By the time Jack returned home from England, his son.      from college.

When I approached to the subway entrance, an explosion occurred. This ___my first experience, I got into panic

— Where is Mr. Wang?— He is not here. He must _______ the students’ papers in the office.

---You’re trembling. What’s happened?---- I ______ outside in the cold for over an hour waiting for a bus.

I insisted that a doctor _______ immediately.