______ time goes by, China is getting stronger and stronger.

I was doing my homework _______ mother was cooking.

The earth will become worse and worse ________ we do something to stop her from being polluted.

—Tom, you look tired this morning. What’s wrong?—I was so busy that I didn’t go to bed _____ 12 o’clock last night.

—What’s the matter with you? You look so worried.-- I don’t know ___________ the problem.