A、$21. B、$9. C、12 D、3 D本题考查的是数字运算能力。对话中男士说“这本书要12美元，但是我只有9美元。”所以如果男士要买这本书就要借3美元。因此，正确答案是选项D。

M: Could you tell me the way to any branch of Bank of China?
W: There’s one two blocks away, near the post office.
Q: Where does the man want to go?

M: The book costs $12, but I only have $9. Could you help me?
W: No problem.
Q: How much does the man need to borrow from the woman?