选择适当的句子填入空白，使对话完整。（其中有两项是多余的）

a. What is it used for?

b. May I ask you a question?

c. Sorry.

d. What is it used for?

e. I think

f. What is it?

g. Do you know

A: Hello, Peter! 1.             

B: Certainly! 2.             

A: Which of these do you think is the most useful invention, a computer, a telephone or a television?

B: 3.             the most useful one is a computer.

A: Why is that?

B: Oh, I think it can help us work and play.

A: 4.             when it was invented?

B: 5.            , I don't know.