Last summer I took a course on _______ poisonous gases.

My mother is a nurse(护士) and is good _____ kids.[ ]

Don’t put the bag _______ on the ground. The things in the bag are easily broken.

Look at that girl. Is it Susan?–No, it _______ be her. She has gone back to her home town .

Tom usually walks to school but sometimes he goes _____.[ ]