He goes to school by bus, but he walks ______after school.

—I don’t know much about computer.—I don’t know,      . Let’s go and ask our computer teacher for help.

You don’t know ______ I want to see you again. It’s a year since I last saw you.

_____, I have to write the words on the paper because she doesn’t know Chinese.

— Do you know sound travels very fast? — Yes. But light travels         sound.