单选题
Did you ever see colored lights in the sky far (1) the earth? Perhaps you saw these beautiful green, yellow, or red streaks (2) in the northern skies at night. Or (3) you have seen sheets or curtains of colored light. Sometimes the lights seem to wave in the wind. It is a truly amazing (4) . You may have seen the aurora borealis. Some ancient people thought the lights (5) from goddesses riding through the sky with shining shields, but astronomers today know (6) . These scientists know that explosions on the sun cause the aurora borealis. From the explosions come (7) pieces of matter which spin around the earth. The small pieces cool and enter the earth's air space and are drawn (8) the North and South Poles. Once inside the earth's air space, these small bits of matter (9) power in the form of light. Though you can see the aurora borealis only (10) , this amazing (11) of color continues during the day as well. Often you can even (12) it. Sometimes it causes strange noises on radio and television sets. The auroras, or bright displays of light, take place near the poles of the (13) . This happens because the poles are like giant (14) . Those tiny bits of matter from the sun are drawn toward (15) . Scientists call auroras near the North Pole aurora borealis. The same kind of light (16) the South Pole is called aurora australis. People living in (17) of Canada, Alaska, and Greenland can see the aurora borealis almost every night. A display of lights may (18) from just a few hours to several nights. Once, in 1958, a giant aurora was (19) from the Arctic down to Cuba. From east to west it stretched 250 miles. And in the other direction, from north to south, it (20) six thousand miles. This huge display was truly beautiful!