A、They travel faster near the TV station. B、They can work better than ever before. C、They usually follow the curve of the earth.

CATV is a short way of saying "community antenna television." But "cable television" is the term most people use. Cable television allows viewers to receive TV programs that they cannot pick up with their regular antennas.
    Television signals do not follow the curve of the earth. They travel in straight lines in all directions. Signals from a TV station move toward the horizon and then go into space. If you live only a few miles from a station, you may not get any picture at all.
    CATV began in 1948. People in places far from TV stations shared the cost of putting up high antennas. A community antenna was usually placed on a hill, a mountain, or on a high tower. The antenna picked up TV signals and fed them into a small local station. From the station, thick wires called cables ran out to nearby homes. Each person using the cable paid a monthly charge.
    CATV worked well, and soon new uses were found for it. Local stations could feed programs into empty channels that were not in use. People along the cable could have local news, weather reports, and farm and school news at no extra charge.
    Today, cable television has moved into cities. It brings in extra programs that city viewers with regular antennas cannot see. It is also used in many classrooms throughout the country.
16. What does the speaker say about television signals?
17. What should people do if they used the cable?
18. What happens after cable television has moved into cities?