______a teacher, John thinks that his duty is to help the students to become better learners.[ ]

The farmer was very thankful to the doctor _________ saved his son．

Liu Yang is the first Chinese woman astronaut ______ has ever traveled in space.

Look at the boy running on the ground. Is it Davis?—It ______ be him. I saw him go to the teacher’s office just now.

How _______ did you do a bird count there?—Twice. [ ]