求翻译：首先，自行车便于携带，其次它也是一种运动，有利于锻炼身体，因为越野自行车的快与刺激，对自己也是个突破和挑战，最重要的是它比汽车节省能源，不污染大气，为环保做出了贡献是什么意思？

First, the bike is easy to carry, and secondly it is also a movement, to exercise, because mountain bike fast and stimulation, is a breakthrough to himself, and challenges, it is the most important than cars to save energy, does not pollute the atmosphere, has contributed to the environmental protection