Horses and zebras can be bred to produce a zorse, which is sterile. which is most likely the reason zebras and horses are considered separate species?

Due to the fact that horses and zebras can be bred to produce any offspring means that they belong to the same genus (Equus). However the offspring that comes about as a result of this cross breeding (zebroid/zorse) is not a fertile offspring. From these results we can conclude that the zebra and the horse are classified into different species. A horse has 64 chromosomes while a zebra has 46 or 32 chromosomes depending on the species. Their offspring often ends up having 54 chromosomes. This difference in chromosomes dictates that the female offspring not sufficiently fertile and the male sterile.

The answer would be sand and gravel