a scientist finds what she thinks is a new species of rodent on a small Pacific island

a scientist finds what she thinks is a new species of rodent on a small Pacific island. However, some similar-looking rodents inhabit some nearby island. She mates the new rodent with the nearby rodents and gets viable but infertile offspring, why?

a. The old and new rodents underwent vicariance, and their hybrid offspring were sterilized by adaptive radiation.

b. The new rodents probably derive from dairly recent ancestors that experienced dispersive allopatric speciation.

c. The new and old rodents are both allopolyploid after undergoing sympatric.