题目内容：
请根据短文的内容，回答题。The Hyper-X
The Hyper-X (超音速飞机) recently broke the record for air-breathing jet planes when it traveled at a hypersonic speed( 超音速 )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 speed using a special sort of engine known as a scramjet (超音速冲压式喷气发动机) .
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 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 ( 助推火箭 ) 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."That brief journey on March 27 makes a major milestone on the way to a new breed of very fast airplanes," says 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 (边缘) of space. Such hypersonic jets could 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 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 traveled at the speed of sound
D．it reached a speed of about 5,000 miles per hour
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