题目内容：
阅读材料，回答题。Short and shy, Ben Saunders was the last kid in his class picked for any sports team．
"Football, tennis, ricket——anything with a round ball, I was useless," he says now with a laugh． But back then he was the object of jokes in school gym classes in England's rural Devonshire．
It was a mountain bike he received for his 15th birthday that changed him． At first the teen went biking alone in a nearby forest． Then he began to cycle along with a runner friend． Gradually, Saunders set his mind to build up his body,increase his speed,strength and endurance． At age 18, he ran his first marathon．
The following year, he met John Ridgway, who became famous in the 1960s for row- ing an open boat across the Atlantic Ocean． Saunders was hired as an instructor atRidgway's School of Adventure in Scotland, where he learned about the older man's cold-water exploits (成就)． Intrigued, Saunders read all he could about Arctic explorers and North Pole expeditions, then decided that this would be his future．
Journeys to the Pole aren't the usual holidays for British country boys, and many peo-ple dismissed his dream as fantasy．"John Ridgway was one of the few who didn't say,"You are completely crazy, '" Saunders says．
In 2001, after becoming a skilled skier, Saunders started his first long-distance expe-dition toward the North Pole． He suffered frostbite, had a close encounter with a polar bear and pushed his body to the limit．
Saunders has since become the youngest person to ski alone to the North Pole, andhe's skied more of the Arctic by himself than any other Briton． His old playmates would not believe the transformation．
This October, Saunders, 27, heads south to explore from the coast of Antarctica to the South Pole and back, an 1800-mile journey that has never been completed on skis．
The turning point in Saunders' life came when __________． A．he started to play ball games
B．he got a mountain bike at age 15
C．he ran his first marathon at age 18
D．he started to receive Ridgway's training
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