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A pure virus (病毒) could be kept in a bottle, just like hundreds of other chemicals. Yet, when a virus is placed on a living thing, it comes to life. As long as it is on a living material, it grows.
A difficult question still remains - what is a virus? Is it living or chemical? Men of science had always thought life and not-life to be as different as black and white. With the discovery of the virus, they became aware of a grey area that was neither black nor white.
Until the 1930’s, it was accepted that there was also a great difference in size between the largest chemical molecules (分子) and the smallest living things. As new and much finer filters (过滤器) were invented, men were able to measure viruses. The first virus to be measured was found to be about 100 millimicrons (毫微米) across.
The largest known chemical molecule measures only 22 millimicrons. The smallest living thing measures almost seven times that size or 150 millimicrons. When viruses were measured, they were found to range in size from 16 millimicrons to 300 millimicrons. Most were found to be larger than the largest chemical molecules and smaller than the smallest living things.
The answer to the puzzle - what is a virus? - must be that it is both living and not living. In a living cell, it is a live thing. In a bottle, it is nothing more than a chemical. We now realize that the virus is actually a link between life and not-life.
What is this text mainly about?
题目问的是“文章大意是什么？”。该题属于主旨题。先要理顺文章各个段落，第一段是引子，引出了第二段所提的问题“病毒是什么?”，然后一直到最后都是对于这个问题的回答，最后一段又重申了这个问题。因此这篇文章主要是说“病毒是什么?”，即我们应该如何去解读病毒的概念。故选A。
What makes a virus come to life?
题目问的是“是什么让一个病毒有了生命？”。该题属于细节题。由第一段第二句“Yet, when a virus is placed on a living thing it comes to life.”可知当病毒附着在一个生命体上，病毒就是有生命的，故选D。
What do the underlined words “a grey area” in paragraph 2 refer to?
题目问的是“第二段中划线词组a grey area指的是什么?”。该题属于推理题。由第二段可知，科学家们总是把有生命和无生命的物体视作是截然相反的两者。随着病毒的发现，他们开始意识到这两者之间还有一个中间的领域，它并不属于任何一方。由此可推断出这里的a grey area(灰色领域)指的是病毒处于一种生命体与非生命体的中间地带。故选B。
补充：a grey area that was neither black nor white这里的black和white分别指的是前面被视作截然相反的两者-life和not-life。
Which of the following is the smallest in size?
题目问的是“以下几个选项中，哪个是尺寸最小的？”。该题属于推理题。
根据倒数第二段最后一句“...larger than the largest chemical molecules and smaller than the smallest living things.”可知，通过三者的比较，应该是chemical molecules最小。故D为正确答案。
补充：注意这里的largest和smallest作的是定语，分别修饰chemical molecules和living things：最大的chemical molecules，最小的living things，而不是说chemical molecules就是最大，living things就是最小。这一点很具有迷惑性。
这里作比较的三者是：the smallest living things>viruses>the largest chemical molecules