17. ---You look worried. What's your______?

  ---I have trouble in learning math.

    A. plan                     B. idea                   C. problem

18. ---I can't find my dictionary.

  ---Is the one on Jack's desk______?

  ---Yes, thank you.

A.his                       B. yours                  C. ours

19. ---What does your sister____?

  ---She is tall and thin.

A. look  like                 B. look for                C. look up