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    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.

中国农业从远古洪荒走来，经历了“刀耕火种”、“牛耕铁犁”等阶段。下列项中属于“刀耕火种”阶段的耕作工具有（   ）

①石斧       ②耧车     ③骨镰       ④曲柄锄