Scientific experiments __________ by students without the teachers’ instructions can be dangerous.

He ___ Beijing for a long time.

Residents ________ not to go out alone or to take part in any groups during the period of recent terrorist activities. 

The interviewer will begin to judge you as soon as he or she ________ you in the waiting room.

 - Will you be available at three o'clock tomorrow afternoon? - No. I ______ a lecture then.