A baby giraffe is born 10 feet high and usually lands on its back. Bringing a giraffe into the world is a tall order. In his book, A View from the Zoo, Gary Richmond describes how a new-born giraffe learns its first lesson.

    The mother giraffe lowers her head long enough to take a quick look. Then she puts herself directly over her child. She waits for about a minute, and then she does the most unreasonable thing. She throws her long leg and kicks her baby, so that it's sent sprawling (四脚朝天).

    When it doesn't get up, what the mother has done is repeated again and again. The struggle (挣扎) to rise is important. As the baby giraffe grows tired, the mother kicks it again. Finally, it stands for the first time. Then the mother giraffe kicks it off again. Why? She wants it to remember how it can get up. In the wild, a baby giraffe must be able to get up as quickly as possible to stay with its group, where there's safety.

    Another writer named Irving Stone understood this. He spent a lifetime studying great people, writing stories about such men as Michelangelo, Vincent van Gogh, Sigmund Freud, and Charles Darwin.

    Stone was once asked if he had found something unusual about these great people. He said, “I write about people who sometime in their life have a dream of something. They're beaten over the head, knocked down and for years they get nowhere. But every time they stand up again. And at the end of their lives they've realized some small parts of what they set out (着手) to do.”

C

    Fat and shy, Ben Saunders was the last kid in his class picked for any sports team. “Foothall, tennis, cricket-anything with a round ball, I was useless,” he says now with a laugh. But back then he was the one always made fun of in school gym classes in Devonshire, England.

    It was a mountain bike he received for his 15th birthday that changed him. At first he went biking alone in a nearby forest. Then he began to-ride the bike along with a runner friend. Gradually, Saunders set up his mind on building up his body, increasing his speed and strength. At the age of 18, he ran his first marathon.

    The following year he met John Ridgway and was hired as an instructor at Ridgway's school of adventure in Scotland, where he learnt about Ridgway's cold-water exploits. Greatly interested, Saunders read all he could about North Pole explorers and adventures, then decided that this would be his future.

    In 2001, after becoming a skillful skier, Saunders started his first long-distance expedition towards the North Pole. It took unbelievable energy. He suffered frostbite, ran into a polar bear and pushed his body to the limit, pulling his supply-loaded sled（雪橇）up and over rocky ice.

    Saunders has since become the youngest person to ski alone to the North Pole, and he's skied more of the North Pole by himself than any other British man. His old playmates would not believe the change.

    Next October, Saunder, 27, heads south from the coast of Antarctica to the South Pole and back, a 2900-kilometer journey that has never been completed on skis.

下列各句中，没有语病的一句是（    ）

①ax²+bx+c=0；②2x（x﹣3）=2x²+1；③x²=4；④（2x）²=（x﹣1）²⑤ $\frac{x^2-3x}{x}$ =2x²．