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 11 powerful enough to shoot astronauts beyond the earth's gravitational pull and 12 them on the moon.We have now become travelers space.

Impressive and complex 13 it may appear, the rocket, which was invented in China over 800 years 14 , is a relatively simple device.Fuel that is burned in the rocket engine changes 15 gas.The hot and rapidly expanding gas must escape, but it can do so only 16 an opening that faces backward.As the gas is 17 with great force, it pushes the rocket in the 18 direction.Like the kick of a gun when it is fired, it 19 the laws of nature described by Sir Isaac Newton when he discovered that“ 20 every action, there is an equal and opposite reaction.”

11.A.shown

B.been

C.appeared

D.proved

12.A.send

B.land

C.take

D.carry

13.A.that

B.so

C.as

D.sometimes

14.A.in advance

B.before

C.earlier

D.ago

15.A.as

B.into

C.for

D.the

16.A.in

B.at

C.by

D.through

17.A.transmitted

B.dispersed

C.erected

D.radiated

18.A.same

B.other

C.opposite

D.wrong

19.A.states

B.proves

C.follows

D.breaks

20.A.like

B.as

C.with

D.for