A、Red. B、Green. C、Yellow. D、Blue. C看到选项都是颜色，就要格外留神听对话中出现颜色的地方。对话两次提到yellow这个词，且未提及其他颜色，故不难得出答案C。

M: Can you tell me something about the university shuttle bus system? This is such a large campus, and I have classes all over campus. I need to take the shuttle bus from one class to another, or I’ll never make it on time.
W: What do you need to know? I think it’s a really great system.
M: First of all, where does it go?
W: The university shuttle bus system goes all over campus. It doesn’t leave the campus; if you want to travel off-campus, you’ll need to take the city bus system. But the university shuttle bus system will get you from one class to the next very efficiently.
M: And how much does it cost?
W: It’s free, can you believe it? So you don’t have to pay a cent to get all around the university campus.
M: That’s really great. And how do I catch the shuttle bus?
W: Just look for one of the bright yellow shuttle bus signs, and go stand next to it. You can see the yellow shuttle bus signs all over campus. A shuttle bus will come along approximately every five minutes, so you shouldn’t have to wait long.
M: That all sounds good. Thanks for your help.
W: No problem.
22. What are the man and woman discussing?
23. What area does the university shuttle bus cover?
24. How much does the shuttle bus cost?
25. What color are the shuttle bus signs?