In Many Ways, a Mule Is a Superior Animal to the Horse

In many ways, a mule is a superior animal to the horse or donkey.Mules are often stronger and can jump higher than either of their parents.Why, then, are mules considered to be a "weak hybrid"?

A) They are sterile.
B) They do not live as long as horses and donkeys.
C) Their offspring are progressively weaker than the parents.
D) They are more prone to disease than are horses and donkeys.