Fat and shy, Ben Saunders was the last kid in his class picked for any sports team. “Football, 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 15^th 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 started to work as a teacher at Ridgway's school of adventure（冒险）in Scotland, where he learnt about Ridgway's cold—waterexploits. Greatly interested, Saunders read all he could about North Pole explorers and adventures; he decided that this would be his future.
    In 2001, after becoming a skillful skier, Saunders started his first long—distance expedition (探险) 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 sled up and over the rocky mountains.
    Saunders has 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.

小华将额定电压相同的两只小灯泡L₁、L₂串联在电路中，如图所示．合上开关时、发现灯L₁亮而灯L₂不亮；当他用一根导线并接在灯L_l的两端时，发现灯L₂亮了而灯L₁不亮．分析出现上述现象的原因是（　　）