wonder why he has been acting so strange these days.—Recent pressure at work may_____ his behavior. 

It is the test system, _____ the teachers, that is to blame for the heavy burden of middle school students nowadays

Look！Our math teacher is standing outside our classroom.—It ________ be him.He has gone to his office.

Under the dome, ______ 103-minute documentary , has brought air pollution to ______ attention of the public.

Jim ______ a late night film at home when, right in the middle a thrilling scene, the television went blank