According to island biogeography, what is the relationship between an island's distance from the mainland and the number of species present on the island?

a. there is no relationship between the distance from the mainland and the number of species found on an island.
b. the farther an island is from the mainland, the larger the number of species found on the island.
c. the closer an island is to the mainland, the fewer the number of species found on the island.
d. the farther an island is from the mainland, the fewer the number of species found on the island.
e. the closer an island is to the mainland, the fewer the number of species found on the island; and the farther an island is from the mainland, the larger the number of species found on the island.