题目内容：
根据材料，完成题。The rocket engine, with its steady roar like that of a waterfall or a thunderstorm, is an impressive symbol of the new space age. Rocket engines have 1 powerful enough to shoot astronauts 2 the earth's gravitational pull and 3 them on the moon. We have
now become 4 space.
Impressive and complex 5 it may appear, the rocket, which was 6 in China over 800 years 7 , is a relatively simple device. Fuel that is 8 in the rocket engine changes 9 gas. The hot and rapidly 10 gas must escape, but it can do so only 11 an opening that 12backward. As the gas is 13 with great force, it 14 the rocket in the 15 direction. Like the kick of a gun 16 it is fired, it 17 the laws of nature 18 by Sir Isaac Newton when he discovered that "19 every action,there is 20 equal and opposite reaction. "
请在第__(1)__处填上正确答案。 A．shown
B．been
C．appeared
D．proved
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