B

    We have all looked up at birds and wished that, like them, we could fly off into the sky. ____56____, we will never he able to fly. And penguins(企鹅). ____57____ they are birds themselves, share the same life. Like us. they have to ____58____ around while other, luckier birds mockingly(取笑地)circle overhead.

    But penguins are birds, aren't they？ So ____59____ on earth can't they fly？ Professor John Speakman, from the University of Aberdeen. asked himself the very same questions. According to BBC, he said, "Like many people. I've always been ____60____ penguins, and seeing them do these marches across the ice, I've often thought, "Why don't they fly？"

    Now, it seems, there is ____61____ to this question. By looking all other kinds of birds, scientists like Speakman are beginning to find out why penguins can't fly. Penguins use their wings to ____62____—they are amazing swimmers. A wing that is good for swimming cannot be good for flying.

    This is called an evolutionary trade-off(进货的权衡). Penguins certainly used to fly. But as time ____63____, they began to find more and more food in the sea. Over millions of years, they had spent so ____64____ time under the waves that they'd forgotten how it feels to fly.

    Don't feed too bad for the penguins. They probably don't even want to fly. ____65____, in many ways the ocean is a better place than the sky could ever be.