Listen! David is talking happily with his mother in the next room. — It ______ be his mother. She has gone to Yunnan to be a volunteer doctor.
The boys ____ a football match tomorrow morning.
I usually sit_____ the classroom, so that I can hear the teacher who likes to stand_____ the class clearly. [ ]
We____here two hours ago . So we___ _here for two hours .
No matter how ______,scientists are eager to know more about it by exploring.