Maybe you know a lot about the city you live in. You may even know some interesting facts about Earth, but 16  do you know about Mars? Will there be  17  there just like people on Earth?

Mars is the fourth planet from the sun and it's the most like Earth, but it's much 18   than Earth because it's farther from the sun. It's also the only planet whose surface can be 19   from Earth.

Mars is often called "the red planet" because the rocks on it have a dark   20   color. There are strong winds on Mars that can 21  dust into the hills.

For many years, people have 22 Mars. They have sent instruments and robots there. Some of the robots got there, 23  most failed.

NASA(美國(guó)國(guó)家宇航局) sent two robots to Mars two years ago. They wanted to  24   signs of water and life on Mars. Some day, humans will travel to Mars and have an even 25  look at our red neighbor. Maybe it could be you!

(   )16.  A. when          B. what          C. which           D. how

(   )17.  A. sound         B. life          C. air             D. water

(   )18.  A. hotter        B. colder        C. bigger          D. smaller

(   )19.  A. seen          B. wrote         C. heard           D. visited

(   )20.  A. red           B. black         C. white           D. yellow

(   )21.  A. blow          B. look          C. mix             D. fly

(   )22.  A. lived on      B. landed on     C. studied         D. written

(   )23.  A. but           B. and           C. however         D. or

(   )24.  A. leave         B. use           C. find            D. give

(   )25.  A. faster        B. farther       C. closer          D. lower