A population of horses is split into two population by a new river bed that forms after a flood. After many generations,the river changes course and the population mix again. Which of the following indicates that the two population have formed two separate species?

Options

A. The population refuse to cross the dry river bed to interbreed
B. The population do not interbreed freely in nature
C. One population has twice as many horses with white spots as others
D. The horses interbreed,but the offsprings are shorter either of the parents