Flying the Hypert Skies A little airplane has given new meaning to the term "going hyper. " The Hyper-X recently broke the record for air-breathing jet planes when it traveled at a hypersonic speed of seven times the speed of sound. That’s about 5,000 miles per hour. At this speed, you’d get around the world -- flying along the equator (赤道) -- in less than 5 hours. The Hyper-X is an unmanned,experimental aircraft just 12 feet long. It achieves hypersonic (超音速的) speed using a special sort of engine known as a scramjet. It may sound like something from a comic book,but engineers have been experimenting with scramjets since the 1960s. For an engine to burn fuel and produce energy, it needs oxygen. A jet engine, like those on passenger airplanes, gets oxygen from the air. A rocket engine typically goes faster but has to carry its own supply of oxygen. A scramjet (紧急刹车) engine goes as fast as a rocket, but it doesn’t have to carry its own oxygen supply. A scramjet’s special design allows it to obtain oxygen from the air that flows through the engine. And it does so without letting the fast-moving air put out the combustion (燃烧) flames. However, a scramjet engine works properly only at speeds greater than five times the speed of sound. A booster rocket carried the Hyper-X to an altitude of about 100,000 feet for its test flight. The aircraft’s record-beating flight lasted just 11 seconds. Although the little plane’s self-powered flight lasted only 11 seconds, that brief journey on March 27 makes a major milestone on the way to a new breed of very fast airplanes, comments Werner J. A. Dahm of the University of Michigan in Ann Arbor. In the future, engineers predict, airplanes equipped with scramjet engines could transport cargo quickly and cheaply to the brink of space. Such hypersonic jets could potentially carry passengers anywhere in the world in just a few hours. Out of the three experimental Hyper-X aircrafts built for NASA, only one is now left. The agency has plans for another 11-second hypersonic flight, this time at 10 times the speed of the sound. The Hyper-X broke the record because

A. it was the first air-breathing jet plane.
B. it flew along the equator.
C. it flew at speeds smaller than five times the speed of sound.
D. it traveled at a supersonic speed.