Statement(S) : A Mule and a liger are not considered to be separate species.
Reason(R) : Mules and ligers are infertile and cannot give rise to their own offsprings.
Both S and R are correct and R is a correct explanation for S
Mule is a sterile offspring produced by a male donkey and female horse. Liger is a sterile offspring produced by a male lion and female tiger. As they are infertile or sterile they cannot produce their own offsprings and hence they are not considered to constitute distinct species.