A horse has a diploid number (2n) of 64 and a donkey has 2n = 62. The hybrid of these two species, mules, have 2n = 63 and are sterile. Why are mules sterile?

A) One chromosome is unpaired and fails to divide properly during meiosis.
B) Mules cannot physically mate with each other.
C) Mules cannot physically mate with horses and donkeys.
D) Horses are not able to form new species.
E) Mules are not true hybrids.