A、Because they have no wings. B、Because they are lighter. C、Because they aren’t well trained. D、Because they are heavy. D

Men have always wanted to fly like birds.(22)Birds can fly easily because they are light, but men’s bodies are heavier.
    Men first went up into the air in balloons. These are big bags, and they are filled with gas.(23)Hydrogen is a useful gas for balloons. It is lighter than air. So balloons are usually filled with hydrogen.
    Balloons have to fly with the wind, as they have no engines to drive them against it. Later, men made airships. These were balloons with engines, but they were not round.(24)They were long, and the engines were at the back. They were also filled with hydrogen and some of them caught fire, because the hydrogen escaped and the engines heated it. Then the airship was completely burnt in a few seconds. People go across the world by aircraft which have engines similar to but more powerful than those of cars. There is another kind of engine which we call the jet engine. An English engineer invented the jet engine. In May 1941 his new engine was fixed in an aircraft, and the aircraft flew quite well. Jet engines are very powerful. Usually two, three or four are enough for an aeroplane; but some big aircraft have six. Anyone in a moving jet plane can feel the power of the engines.(25)Jet planes can travel faster than sound. As a flying jet plane leaves its noise behind, we do not hear it until it has gone.