A

When you look at the sky at night，the moon looks bigger than the stars． In fact，the moon is much smaller than the stars．It is much smaller than the earth． But the moon is much closer to us than any star． That is why it looks so big． If you hold a coin close．to your eye，it looks big．If you look at it across the room，it looks small．The moon moves around the earth． It makes one trip in about four weeks． The moon looks flat(扁的)to us． But it is a round ball，like the earth．People once thought the moon had fire(火) on it． They thought the fire made it bright．Now we know the moon is like a mirror( 镜子 )． It gets its light from the sun Our sunlight comes from the sun，too: What is the sun? The sun is star． The stars we can see have their own light． There are many big stars we can-t see． Their light has burned out． Others are still bright，but they are so far away that we can-t see them． The sun looks bigger and much brighter than other stars because it is the nearest of all stars． The sun and the other stars we see are very hot，but the air around us saves us from(使------免于) the heat(热量) of the sun．The sun gives us light and warmth．It makes plants grow and leaves turn green． It makes life possible on our earth．It is a life-giving star．

(      )21． The moon looks bigger because__________ ．