Which one of the following is the correct statement about the respective animal(s)?
(a) Donkey and horse should be considered one single species because they can successfully mate and produce an offspring.
(b) Donkey and horse are two different genera.
(c) Mule is a separate species.
(d) Mule is neither a donkey nor a horse.
_________ is a cross between a male donkey and a female horse.