We all know trees play an important role in _______up the air.
| A．clean | B．cleaning |
| C．cleaned | D．Cleans |
There is no _____ that phones play an important role in the modern life.
A.question B. doubt C. problem
There ______ more new buildings in my hometown in two years .
A. are B. will haveC. will be
The little boy is healthy, _____ he is happy, too.
A.and B. but C.or
The meeting for two hours.
A. has begun B. has been onC. began
Teachers should _____ their students to practice _____ English often.
A.encourage; speaking B.let; speak C.keep; speak