Columbus was able to convince ferdinand and isabella to support his voyages because
a. based on his mathematical calculations of the earth's circumference, he believed that sailing west from europe to asia would be profitable and they recognized the wealth such a route would bring to their kingdom
b. he promised them an alliance between the wealthy italian port city of genoa and the rising newly unified spanish kingdom of aragon and castile
c. he had already made a successful voyage to the islands he called the indies and he had the gold and natives as specimens to prove it
d. he had had the earlier backing of prince henry who died before he could back another voyage