The universe is really mystical（神秘的）．Scientists have done lots of study about it but there is till much they haven't discovered．

Scientists think that there （41） 　　life on the earth for many many years．However，we haven't found life on any other planets （42） 　　．

There are eighteen planets going around the sun．The earth is one of them．There are humans，animals，plants and many other kinds of life on the earth．There is water and air，too．And compared to the other planets，the earth is not too （43） 　　or too cold．None of seven other planets has an environment like that of the earth．

The sun and its planets are called the solar system，and our solar system is a small part of a （44） 　　larger group of stars and planets，called the Galaxy or the Milky Way．There are （45） 　　stars in the Galaxy，and our sun is only one of them．

Scientists have also discovered many other galaxies in the universe．Their light has to travel for many years to reach us．So （46） 　　is the universe？it is impossible to imagine．

Scientists have sent spaceship to the planet Mars （47） 　　photos．they have even sent spaceships to travel outside （48） 　　．However，no spaceship has traveled （49） 　　to reach other stars in our Galaxy．

When we see stars twinkling（闪耀） in the sky at night，we may think a lot：（50） 　　so many stars in the universe，is there life out there in space？If there is life on other planets，what life will be like there？What transports do they use to travel？why don't they visit us？We do not know the answers…

41．A．is going to be	B．are going to be	C．has been	D．have been
42．A．yet	B．already	C．still	D．just
43．A．cool	B．warm	C．hot	D．wet
44．A．most	B．more	C．many	D．much
45．A.200 billions of	B.200 billion of	C．billions of	D．billion of
46．A．how large	B．how far	C．how wide	D．how long
47．A．taking	B．to take	C．take	D．took
48．A．Mars	B．the sun	C．the earth	D．the solar system
49．A．enough far	B．far enough	C．enough close	D．close enough
50．A．with	B．without	C．by	D．on．