[听力原文]16-20Television presents a vivid world in front of us. For example, TV can keep us informed of what is happening throughout the world. For another example, with a TV set at home. we can visit a great deal of wonders of the world in a day, even in an hour, the Great Wall of China, the skyscrapers in New York, the pyramids in Egypt and so on. Besides, TV brings us various forms of entertainment, such as sports competitions, concerts, etc. Television has become one of the most influential tools of communication today. It affects the life of everyone from the infant to the adult. The influence of this powerful medium ranges widely from most beneficial to extremely harmful. Then how to select the programs is critical. What is critical？