21. Now scientists are doing their best to make _______ breakthrough in cancer research.

A. the           B. /             C. a         D. one

22. Some diseases ________ quickly from one person to another.

A. move       B. catch          C. spread      D. expand

23. — Where have you been? We _____ you everywhere.

— Sorry to have made you worry about me.

A. have been looking for  B. looked for     C. had looked for           D. look for

24. In Chinese, the same pronunciation with different tones has different meanings. The same is _______ music.

A. true of        B. similar to      C. true to         D. similar with