They have different body sizes. A mule is not able to reproduce. They are found in different environments. A mule does not resemble either parent.
I gave my students this multiple choice question: A donkey and a horse can be bred to produce a mule. Which observation is evidence that donkeys and horses are two different species?

Only one answer is correct among these 4 choices:
- They have different body sizes.
- A mule is not able to reproduce.
- They are found in different environments.
- A mule does not resemble either parent.

Could you tell me which one is correct?

A mule is not able to reproduce.