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 【C1】______ powerful enough to shoot astronauts 【C2】______ the earth's gravitational pull and land them on the moon. We have now become space 【C3】______ . Impressive and complex as it may appear, the rocket, which was 【C4】______ in China over 800 years ago, is a relatively simple device. Fuel that is 【C5】______ in the rocket engine changes 【C6】______ gas. The hot and rapidly 【C7】______ gas must escape, but it can do so only through an opening that 【C8】______ backward. As the gas is 【C9】______ with great force, it 【C10】______ the rocket in the 【C11】______ direction. Like the kick of a gun 【C12】______ it is fired, it 【C13】______ the laws of nature 【C14】______ by Sir Isaac Newton when he discovered that' 【C15】______ every action, there is an equal and opposite reaction.' 【C1】______