A、Intelligent life. B、Canals of some kind. C、Signs of life. D、Natural resources. A

Astronomers have spent hundreds of years searching for signs of life on other planets, using telescopes. When they saw the geography of Mars, they thought they could see canals and that this might be evidence of intelligent life on the planet. More recently, however, spacecraft have been sent to analyze the soil for signs of life. The results were negative and astronomers are now convinced that no life exists on the surface of any other planet in our solar system. To send a spacecraft far beyond our solar system is not realistic because of the huge distances involved.
    The only way we are likely to know of its existence is from radio messages unless life comes and visits us. Listening for intelligent life is not a new idea, but the techniques now being used to offer much better chance of success than before. Looking for evidence of life in other solar systems now means using special radio receivers called radio telescopes to listen out for messages. The assumption is that intelligent forms of life would have discovered radio waves and would, like us, be using them to communicate.
    Through its research work, astronomers have developed two means of systematically searching for intelligent life. The first method is to select the closet 800 stars like our sun and to direct a radio telescope to scan each one of them for electromagnetic waves. The second way is to survey the entire sky, listening for continuous signals. All this, as well as the chance of discovering that maybe we are not alone in the universe!