树上原来有4只小鸟，飞来2只，现在有____只；又飞来2只后，一共有____只。

    Alice is a white dog. She is two. Bob is a brown dog. Bob and Alice are good friends. Look at the two dogs.

Bob: Good morning!

Alice: Good morning!

Bob: What's this?

Alice: It's a jacket. It's my sister's. Oh, my sister is Grace.

Bob: What's that?

Alice: It's a book. It's Miss Miller's book. She is an English teacher.

Bob: Well, dogs don't like books.

You must leave now ________ you can meet your mother at the airport on time.