The reason why the laser beam is very strong is th

单项选择题

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. And that was where we stuck, in surgical instrument terms, for many years. In the 1960s a new tool was developed, one which was, first of all, to be of great practical to the armed forces and industry, but which was also, in time, 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, for a very large number of different complaints. The word laser means: light amplification by stimulated emission of radiation. As we all know, light is hot; any source of light--from the sun itself down to a humble match burning --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 effct on certain living cells. It is now possible for eye surgeons to operate on the back of the human eye without harming the front of the eye, simply by passing a laser beam right through the eyeball. No knives, no stitches, no unwanted damage--a true surgical wonder. Operations which once left patients exhausted and in need of long periods of recover time now leave them feeling relaxed and comfortable. So much more difficult operations can now be tried.
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 more effective. Altogether, tomorrow may see more and more information coming to light on the diseases which can be treated medically.

The reason why the laser beam is very strong is that ______.

[A] it is made up of a concentrated beam of light

B. it sends out heat in all directions

C. it is artificially illuminated

D. its heat is increased by the heat of the sun