What Does the Theory of Island Biogeography Attempt to Illustrate

What does the theory of island biogeography attempt to illustrate?

A) Why islands have fewer species than the mainland
B) Why small islands always have the same number of species no matter where they are
C) Why species migrate to some islands more than others
D) Why bigger islands tend to have relatively fewer species than smaller ones
E) What factors determine the numbers of species on islands