Betty: Hey, Tony! How was the UK?
Tony: Guess what! I saw Sally in London.
Guess what! = Guess who I saw?
Lingling: My pen friend Sally?
Tony: Yes, Sally, the girl who visited us last year and played in the orchestra.
who 引导定语从句，修饰先行词the girl.
Lingling: Oh, that’s fantastic! How is she?
