Ptolemy improved the geocentric model by  A) changing the circles to ellipses B) moving the Sun to the center C) adding epicycles and deferents D) adding crystalline spheres  

Answer:

C) adding epicycles and deferents 

Step-by-step explanation

There are two circles for planets which rotate around the Earth according to the geocentric model. The two two circles are small and large circles. The smaller one is termed as the epicycle. The larger one is termed as the deferent. The small epicycles are attched to the large circle deferents centered on Earth. the retrograde motion may present when the small epicycle moves around the deferents at the point towards the Earth. 

Based on this epicycles and deferents, the retrograde motion of Mars could be detected by Ptolemy. Then he improved his geocentric theory by adding epicycles and deferents.