单项选择题

Directions: There are 10 blanks in the following passage. For each numbered blank, there are 4 choices marked A, B, C and D.

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 (76) powerful enough to shoot astronauts (77) the earth’s gravitational pull and (78) them on the moon. We have now become (79) in space. Impressive and complex (80) it may appear, the rocket, which was (81) in China over 800 years (82) , is a relatively simple device. Fuel that is (83) in the rocket engine changes (84) gas. The hot and rapidly (85) gas must escape, but it can do so only through an opening that faces backward. As the gas is radiated with great force, it pushes the rocket in the opposite direction.