补全对话，根据所给对话，填写适当的内容，使其完整、正确。(每空词数不限)。

A: Jack, is this your book?

B: . It's his book.

A:

B: It's a key.

A: ?

B: K-E-Y.

A: Is this ?

B: No, it isn't. It's my sister's key.

A: Thank you.

B: