如图，AB是⊙O的弦，OC⊥AB于点C，若AB=4，OC=1，则⊙O的半径为（　　）

    The earth moves round the sun, and the moon goes round the earth. When our part（部分）of the earth turns to the sun, it is day, When our part of the earth turns away from the sun, it is night. The sun is much bigger than the moon. But sometimes the moon looks bigger than the sun, because it's much nearer to the earth. The sun is very bright. It gives a very strong light. The moon looks quite bright, too. But it doesn't give any light at all. The moon looks much bigger and brighter than the stars. But actually（事实上）, the stars are much bigger and brighter than the moon. They look smaller than the moon because they're much farther away from us.