写一篇关于人的能力(man's ability)的作文，要求：
1．人类比动物优越；
2．人类所取得的成就；
3．人类所面临的挑战。

[范文] 
人的能力(man's ability)
    when man first appeared on earth, he had advantages over other creatures. he had a large brain, an upright body and a pair of clever hands. these enabled him to make tools and make fire, which is vital in his struggle for existence. furthermore, man invented languages which allowed him to communicate with his fellow men. up to the present, man has made tremendous achievements. he has invented all kinds of means of transportation to travel from place to place. he has conquered many diseases and prolonged his life span. he has also developed such new scientific and technological tools as laser, computer, not to mention the use of nuclear power and the landing on the moon. so there is no limit to man's ability to create miracles. however, man is also faced with many challenges. the biggest is population explosion. with so many mouths to feed, food production can hardly keep up with population increase. next comes the exhaustion of natural resources. man has to think of the prospect that some day the world may run out of water and energy. finally, man has to fight against enviromnental pollution for a clean world.