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.