Darwin observed that tortoises of the nearby islands, although slightly different, belonged to the same species. How might he have known they were the same species?

A) He probably compared DNA sequence data from the different tortoises.
B) He probably compared the metabolism of the different tortoises and observed similar enzymes.
C) He may have heard that tortoises of different islands could mate with each other.
D) He may have moved one group of tortoises to a new island to observe accumulated changes.