Scientists think that there has been life on the Earth for millions of years．However，we haven't found life on other planets yet．

The Earth is a planet and it goes around the Sun．Seven other planets also go around the Sun．None of them has an environment with air，so people and things cannot grow．The Sun and its planets are called the solar system （太阳系）．The solar system is a small part if our galaxy （星系）．

Scientists have also discovered many other galaxies in the universe．But they are a long way away because their light has travelled for many years to reach us．It is hard to understand how large the universe is．

So far，scientists have sent lots of spaceships to look at other planets in our solar system，and some spaceships have gone beyond the solar system．However，no one has discovered any life on other planets yet．

51．How many planets go around the Sun？　　

A．Seven．

B．Eight．

C．Nine．

52．In order to look at other planets in our solar system，scientists have sent many．　　

A．e﹣mails．

B．planets

C．spaceships．

53．has found life on other planets．　　

A．No one．

B．A scientist．

C．A traveler．

54．Which of the following is TRUE？　　

A．All the planets have an environment with air．

B．The solar system includes the Sun and its planets．

C．There is only one galaxy in the universe．

55．The best title of this passage should be．　　

A．The Earth

B．Our solar system

C．The stars at night．