从方框中选择合适的句子补全对话。

A．Is that your pen? B．What a nice pen! C．It's a pen. D．It's red and blue. E. It's under the desk.

Mary: Hello, Tom. What's that on the desk?

Tom: .

Mary: .

Tom: Yes, it is.

Mary: What colour is it?

Tom: .

Mary: .

Tom: Thank you!

Mary: Tom, where is your ruler?

Tom: .