—Long time no see. Haven’t you graduated from college?—Yes. I _______________Social Science for 4 years in Shanghai.
The boy is waiting ____ his sister and they will go to the hospital to wait ____ their sick mother.
Fresh air is good ____ your health.
It's very kind____ you to repair the bike ____ me.
We go to school ____ a bike.
It happened ____ the Long March.