At that time everyone believed that the earth was at the centre of the universe. But Copernicus realised that this picture did not agree with astronomical observations. He found out that the Sun was at the centre with all the planets moving around it. He said that our earth takes a year to travel around the Sun. It revolves once every 24 hours. Copernicus believed that the planets moved around the Sun in perfect circles. Fifty years later, Kepler used the extremely accurate measurements of Tycho Brahe to show that they do not. Copernicus's belief that the Sun was at the centre of the universe was based on:

• A.  common sense.       
• B.  his astronomical observations,
• C.  mathematical calculations.        
• D.  observations of other astronomers,
• E.  None of these