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.