单项选择题

If you are travelling in a train, the things (Example: (0) ) seem to be (41) backwards. But this is not really true. It is the train that is moving.
It is like the case (42) the earth and the sun. The sun seems (43) in the east, move across the sky, and set in the (44) . In other words, the sun seems to travel round the earth. However, (45) not really true. It is the earth that is turning on (46) own axis (轴) . The turning of the earth on its own axis is what makes the change (47) day to night.
The earth also travels round the sun on an unchanging path (路径). As well as the earth, (48) other planets(行星) of (49) sizes. These are known as the solar system. The planet nearest the sun is Mercury, and the planet (50) from the sun is Pluto.