must be found to replace coal．彻底改变对不可再生资源的非理性滥用是绝对有必要的。例如，必须开发节能的机动车减少汽油的消耗量，并且必须找到可替代能源取代煤。

　　48. While achieving success is easier said than done , persistence does in fact pay off . One of the most important traits of a successful person is self-confidence , another is desire , and still another is determination．获得成功说起来比做起来容易，然而坚持不懈确实会有好结果。成功人士的最重要的特征之一是自信，第二是渴望，还有一个是决心。

　　49. Recognizing a problem is the first step in finding a solution．认识到问题是找到解决办法的第一步。

　　50. Many of the explanations offered thus far are at least to a certain extent valid , but none fully address the problem and the issue must be examined in a wider context．目前提供的许多解释至少在一定程度上是正确的，但是，没有一个解释能完全处理问题，这件事情必须放在更广阔的背景中考虑。