Text 4 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 makes a virus come to life（）

A large chemical molecule.
B. A dead chemical.
C. An active virus.
D. A living cell.