Why didn‘t Caroline recognize the customer straight away ?

I bought two expensive dictionaries here half an hour ago, 

but I forgot to take them with me.

Who served you, sir?

The lady who is standing behind the counter.(柜台)

Which books did you buy？

The books whick are on the counter.

Did you serve this gentleman half an hour ago, Caroline?

He said he is the man who bought these books.

I can‘t remember.

The man who I served was wearing a hat.

Have you got a hat, sir?

Yes, I have.

Would you put it on, please?

All right.

Is this the man that you served, Caroline?

Yes.

I recognized him now.

新概念学习（121）The man in a hat