I met my teacher _____ I was walking in the street, but I didn’t say hello to him.
The young man was often seen _____ by the lake.
Remember to spend some time _____ your loved ones, because they’re not going to be around forever.
I think ____ of the materials I listened to at the beginning of the exam ____ easy.
A little boy wrote _____ “u” and _____“n” on the wall.
--- Why do you often take exercise? --- _____ me keep fit.