从所给出的单词中选出合适的完成对话。

What's    Let's    can't    first    pupil

Pat:____the matter?

Tim: I am sad today. I____ run fast. I'm not a good____.

Pat: That's not true.

Jane: Come on, Tim.____ run.

Pat: Go and try, Tim.

Jane: Wow! Look at Tim! He is fast!

Tim: I'm ____! I can run fast.