【单选题】

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 (61) powerful enough to shoot astronauts (62) the earth’s gravitational pull and land them on the moon. We have now become space (63) .

Impressive and complex as it may appear, the rocket, which was (64) inChina over 800 years ago, is a relatively simple device. Fuel that is (65) in the rocket engine changes (66) gas. The hot and rapidly (67) gas must escape, but it can do so only through an opening that (68) backwarD、As the gas is (69) with great force, it (70) the rocket in the (71) direction. Like the kick of a gun (72) it is fired, it (73) the laws of nature (74) by Sir Isaac Newton when he discovered that" (75) every action, there is an equal and opposite reaction.\
A．that
B．when
C．if
D．although