Thousands of years ago man used handy rocks for his surgical operations. Later he used sharp bone or horn, metal knives and more recently, rubber and plastic. In the 1960s a new tool was developed, one which was, first of all, to be of great practical use to the armed forces and industry, but which was also to revolutionize the art and science of surgery. The tool is the laser and it is being used by more and more surgeons all over the world. As we all know, light is hot, and any source of light will give warmth. But light is usually spread out over a wide area. The light in a laser beam, however, is concentrated. This means that a light with no more power than that produced by an ordinary electric light bulb becomes intensely strong as it is concentrated to a pinpoint-sized beam. Experiments with these pinpoint beams showed researchers that different energy sources produce beams that have a particular effect on certain living cells. It is now possible for eye surgeons to operate on the back of human eye without harming the front of the eye, simply by passing a laser beam right through the eye-ball. Operations which once left patients exhausted and in need of long period of recovery time now leave them feeling relaxed and comfortable. The rapid development of laser techniques in the past ten years has made it clear that the future is likely to be very exciting. Perhaps some cancers will be treated with laser in a way that makes surgery not only safer but also more effective. After the use of the laser beam, surgeons can perform operation which

A. leave their patients with negative effects
B. can treat only human eye diseases
C. do little damage to their patients
D. make their patients need a long time to recover