【正确答案】 A

【答案解析】[听力原文]
What we see when we photograph the sun is a huge glowing ball of gas. Underneath this hot and extremely bright photosphere lies other gases. While, in the central parts of the sun, atoms are continually being broken down and built up. In the center of the sun it is just as if hundreds of thousands of hydrogen bombs were continually exploding, and the result of this reaches the photosphere. This is why the photosphere shines so brightly and sends out x-rays and ultra-violet light. Yet sometimes dark spots appear on the photosphere and these "sunspots" are evidence of greatly increased activity below. When sunspots appear, great bursts of the longer radio waves are often received. These are a thousand times more intense than those which radio-astronomers normally receive and which come from what is called the "quiet" sun. These immense bursts of radio waves are usually accompanied by an intensely bright area of "flare" near a sunspot, and it is clear that they are caused by a sudden burst of thermal radiation from below the photosphere. The visible flare and the burst of radio waves are also accompanied by a burst of x-rays, ultraviolet rays, and atomic particles.
When there are sunspots but no flares accompanying them, the radio waves emitted by the "quiet" sun do not remain constant but begin to fluctuate, and the radio signals gradually become stronger. These changes do not correlate with changes in the number of sunspots, although it has been found that the radio waves seem to be generated in the sun"s atmosphere above the sunspots and not in the sunspots themselves. The changes in radio signal do, however, correlate with the calcium areas of the sun"s spectrum.