阅读下列短文，从各题所给的A、B、C和D项中选出最佳选项。
Two men were sitting together in an airplane. They were on a long journey. One of the men was a teacher. The other was a cook.
They sat without talking for a while, then the cook said, "Let's do something to pass the time."
"What do you want to do?" the teacher asked. "We can ask each other riddles (谜语)," the cook said, "You start."
"Let's make the rules first," the teacher said. "To make the game more interesting, we play it for money. If we don't know the answer to a riddle, we have to pay a dollar."
The cook thought about it and said, "That's not fair (公平的). You are a teacher, an educated man. You know more things than I do. I am just a cook."
"That's true," the teacher said, "What's your suggestion?" "If you don't know the answer to a riddle, you need to pay me $100. If I don't know the answer to a riddle, I'll pay you $50." the cook said.
Thinking for a while, the teacher said, "OK. That's fair. Who'll go first?"
"I will," the cook said. "Here is my riddle. What has three legs when it walks, but only two legs when it flies?"
The teacher repeated the riddle, "What has three legs when it walks, but only two legs when it flies? Mm, that's a good one. I'm afraid I don't know the answer."
He gave the cook $100, then he said, "What's the answer?", "I don't know," the cook said and gave him $50.
