(    )20.Jack ________his homework when someone knocked at the door.

      A. is doing     B. was doing      C.  did      D.will do

A long time ____people thought that the sun went round the earth .In some countries people even said that the ____was a god . They thought the god drove across the sky each day on a golden horse. Now we know the earth goes round the sun . It takes the earth a year_____all way round the sun . Today men even know how far ____for the earth to go round the sun .They tell us that the earth travels over a thousand miles a minute on its journey round the sun.

The sun is really a star . It is much ____than the earth and it is very ____. Some people have been to the moon but we know that ____can ever go to the sun . It is far too hot _____people to live ____near it . The sun is _____times heavier than the earth and more than million times larger.