【正确答案】[范文]
The 20th century has seen many developments in science and technology. The widely-used computer serves as the most notable example: Only a few decades ago, people knew very little about computers, whereas now nearly every work unit or family, at least in cities like Beijing, has one or more computers.
Computers have greatly changed our life. To begin with, they have made our work easier. Scientists use them to do all sorts of complicated calculations. Ordinary people gain easy access to the information they want through the use of computers. Even children benefit from computers, which can help them with their studies.
Secondly, computers have made our time more flexible. Instead of our work being confined to a certain period of time and to a fixed place, people can now work while travelling or simply work at home. This flexibility makes our working lives not only less stressful but also more efficient. Thirdly, computers have made our life more interesting. We can now make friends with people all over the world through the Internet, and feel that we are now living in a global village.
However, as the old saying goes, "Every coin has two sides", computers also give rise to a number of problems. They may refuse to work if there is a breakdown because of a virus or a technical failure. When people become too dependent on computers, they will not be able to function when computers are not available. Worst of all, children may get harmful information via the Internet or become addicted to computer games. All these bad effects call for our special concern.
All in all, I would say that computers bring us more benefits than problems. If properly used, computers are essential for improving life in the 21st century.

【答案解析】[解析] 这道题本质上就是一道“利弊型”的题目，但命题的语言却为考生设置了几个障碍，一是考生要从题目列出的20世纪的发明中选择一种，要么是写计算机，要么是写核电力，不需要两种都写，也不要写其他的发明。第二个障碍是题目问这个发明对人类生活的影响是否总是积极的，所以在写的时候考生就不能在积极的和消极的影响上花费同样的笔墨，应当要有所侧重。根据题目的要求，文章中积极方面的内容应当占大约70%，消极的方面最多占30%。这样才能保证文章在主体上是肯定的，在很小的程度上表示否定。因为题目是问“这项发明对人类的影响是否一定是肯定的”，这种提问的方式显然已经先确定它对人类的影响应当是肯定的。
这篇文章具体应当怎么写呢，我们以计算机为例：第一段可以谈一下20世纪科技有许多进步，计算机的广泛使用就是一个最显著的例子，然后描述现象，几十年前，人们对计算机几乎是一无所知，而现在许多家庭都已经拥有计算机，它已在很大程度上改变了我们的生活。接下来在第二段写计算机对人类的积极的影响，它使工作更轻松，这里可以举几个例子，例如科学家用计算机进行复杂的运算，普通老百姓可以通过计算机很容易就找到所需要的信息，孩子们使用计算机来帮助学习。第三段再从另一个角度写计算机的积极的影响：它可以使人们更灵活地安排自己的时间。人们不需要再固定在一个时问和一个地点上班，通过计算机可以在任何地方工作。时间安排就可以更加灵活，生活没有以前那么紧张，工作也更有效率了。主体段落中还可以继续讲计算机使人们的生活更加有趣，可以通过因特网交朋友等等。
写完这些积极的影响后一定要用一个自然段来写消极的方面，计算机也有一些不良的影响，但这一段不需要展开，因为要在文章的主体上达到肯定的效果，因此大致地点几个现象就可以了，例如孩子可能会通过因特网得到一些有害的信息，人们过分依赖计算机，病毒或技术上的失误可能导致一些重要资料丢失。要注意在这一段的任何一个现象都是点到即止，不需要再深入阐述。
文章的最后一段自然地得出结论：计算机的利大于弊，如果使用得当，计算机对人们在21世纪提高生活质量会起到很重要的作用。