从A、B、C、D四个选项中选出能填入空白处的最佳答案。
Is there anything strange on the top of the world's highest mountains? If so, is it a bear? Is it a monkey? Or is it a kind of man?
No one 1. The world has tried their best to know more about this mystery (神秘) for years. In 1887, 2 mountain climber found very big footprints of a very large man. 3 man doesn't walk barefoot（光脚）in the snow. In 1906, 4 climber saw more than footprints. To his surprise, he saw a great hairy animal 5 on two legs. As he watched, it ran out of sight（视线）.
Fifteen years later, there were 6 about the thing in the newspapers. A mountain climber said that he had seen the “snowman” 7 slowly across the snow, far below him. He said it looked like a very large hairy man.
From that time on, more and more people had stories to tell. But not until 1951 did a mountain climber bring back 8 of giant (巨大的) footprints. The picture showed 9 that the snowman walked on two legs. So it was not a bear or a monkey. Could it be an ape man? The mystery grew!
And the mystery keeps on growing. 10 men still hunt the snowman in the high mountains. Some day we may find out just what it is that makes the giant footprints.
的图象过原点且它的导函数
的图象是