(1)
A)Over 100 years.
B)A century or so.
C)Less than 100 years.
D)It is not stated in the passage. (2)
A)They are impossible to guide.
B)They may go up in the flames.
C)They tend to leak the gas.
D)They are cheaply made. (3)
A)Only in the morning.
B)During the early afternoon.
C)In the early morning or the late afternoon.
D)In the early afternoon or late at night.
原文：
Balloons have been used for sports for about 100 years. There are two kinds of sport balloons. One is gas, the other is hot air. Hot air balloons are safer than gas balloons because gas balloons may catch fire. Hot air balloons are preferred by most balloonists in the United States because of their safety. They are also cheaper, and easier to manage than gas balloons. Although pilots feel confident in operating a balloon, they must watch the weather carefully. The best time for sport balloon flights is early in the morning or late in the afternoon, when the wind is light. Over the years, balloonists have tried to cross the Atlantic Ocean. It was not until 1978 that three American balloonists succeeded in doing this.
Questions 18 to 20 are based on the passage you've just heard.
18. For about how many years have balloons been used for sports?
19. Why are gas balloons considered dangerous?
20 At what time of day do most balloon flights take place?