A、Balloons are easier to lift. B、It is a good time for exercise. C、The visibility is good. D、There is no danger in flying. C

Balloons consist of a large flexible bag containing either hot-air or the gas that is lighter than air. The bag no neither envelope is made of silk, rubber or other suitable materials. Piloted balloons carry one or more persons; unpiloted balloons are usually used for scientific research and carry instruments to measure and record a variety of physical phenomena. Since the first successful manned balloon flied in 1783, hot-air balloons have been flown primarily for pleasure. Although the military made extensive use of them in World War I, occasionally rised for sport, the colorful globes are a more commonly thing flying in the sky during the summer when visibility is high and the flights are generally peaceful. Any gas that is lighter than air can be used to lift the balloon. Natural gas, heated air and manufactured gas can and have been used to fly balloons. The earliest balloons were filled with hot-air and often carried a metal container for burning coal to continuously heat the air. Modern balloons are usually filled with hydrogen or air heated by a small gas burner. Balloons are used primarily for two purposes, sport or scientific research. Sport balloons mostly use hot-air, they range from 10 to 20 meters in size when the bag’s inflated; scientific balloons generally use hydrogen and some particular gases. They range in size from 30 to 200 meters when fully inflated.
Questions:
11. The common use of unpiloted balloons is______
12. Why do people usually fly hot-air balloons in summer?
13. What enabled the earliest balloons to fly?