Why was the Copernican model a controversial proposition? Select the three correct answers.
A): It contradicted established scientific understanding.
B): It was mostly incorrect.
C): It demoted Earth to just another planet.
D): It was a complex model.
E): It was not supported by observations.
F): It contradicted religious doctrine of the time.

A): It contradicted established scientific understanding.

C): It demoted Earth to just another planet.

F): It contradicted religious doctrine of the time.

Explanation:

While Ptolemy's model of the universe (the accepted model at that time, since Ptolemy was a respected astronomer and geographer) placed the Earth motionless in the center of the known universe; Copernicus's model of the universe (heliocentric theory) refuted this geocentric theory proposed by Ptolemy and accepted by the Catholic Church.

In this sense, Copernicus proposed the Earth orbits the Sun and rotates once a day on its own axis.  In fact, he also dared to affirm that the other planets also orbited the Sun. This meant the Earth was no longer unique, nor did it occupy the center of the known universe.  

Of course this theory was not accepted by the Catholic Church, since these ideas contradicted the established scientific and religious understanding of the Middle Ages.