By and by, _____ go out to contact other people in person, more and more people tend to socialize with others via electronic devices.

A. other than    B. rather than

C. more than     D. less than

By careful observation and precise calculation,  Nicolaus Copernicus _____ that there is no one center in the universe and the sun is the center of the solar system.

1. A.
   believed
2. B.
   thought
3. C.
   concluded
4. D.
   imagined

________ by a crowd of people, he felt nervous, ________ what to say.

1. A.
   Watching; didn't know
2. B.
   Having watched; and not know
3. C.
   Having been watched; not known
4. D.
   Being watched; not knowing

By accident, he _______he had done something wrong and that he hadn’t been prison for a few weeks.

A. let out   B. made out    C. made sure     D. took care

By the time the fire-fighters arrived, the fire had been put＿＿＿by the teachers and the students.

A. down     B. away     C. out     D. off