单选题Directions: Read the following text. Choose
the best word or phrase for each numbered blank.
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
{{U}} {{U}} 1 {{/U}} {{/U}}powerful enough to shoot astronauts
{{U}} {{U}} 2 {{/U}} {{/U}}the earth's gravitational pull and
{{U}} {{U}} 3 {{/U}} {{/U}}them on the moon. We have now become
{{U}} {{U}} 4 {{/U}} {{/U}}in space. Yuri Gagarin from Soviet
Union was the first human to visit space on April 12, 1961. In the 108 minutes'
journey, he covered 40,000 kilometers. China's first astronaut into space is
Yang Liwei. Impressive and complex {{U}} {{U}} 5 {{/U}} {{/U}}it
may appear, the rocket, which was {{U}} {{U}} 6 {{/U}} {{/U}}in
China over 800 years {{U}} {{U}} 7 {{/U}} {{/U}}, is a
relatively simple device. Fuel that is {{U}} {{U}} 8 {{/U}}
{{/U}}in the rocket engine changes {{U}} {{U}} 9 {{/U}} {{/U}}gas.
The hot and rapidly {{U}} {{U}} 10 {{/U}} {{/U}}gas must escape,
but it can do so only {{U}} {{U}} 11 {{/U}} {{/U}}an opening
that {{U}} {{U}} 12 {{/U}} {{/U}}backward. As the gas is
{{U}} {{U}} 13 {{/U}} {{/U}}with great force, it {{U}}
{{U}} 14 {{/U}} {{/U}}the rocket in the {{U}} {{U}} 15
{{/U}} {{/U}}direction. Like the kick of a gun {{U}} {{U}} 16
{{/U}} {{/U}}it is fired, it {{U}} {{U}} 17 {{/U}} {{/U}}the
laws of nature {{U}} {{U}} 18 {{/U}} {{/U}}by Sir Isaac Newton
when he discovered that "{{U}} {{U}} 19 {{/U}} {{/U}}every
action, there is {{U}} {{U}} 20 {{/U}} {{/U}}equal and opposite
reaction." Newton's theories have contributed a great deal to the science of
aeronautics.