By the 15^th Century the Portuguese Dominated the Western Water

By the 15^th century the Portuguese dominated the western water route to Asia because

A) they monopolized navigational technology which remained unknown to the rest of the world for many centuries.
B) they refused to cooperate with the Catholic Church with respect to conversions and could concentrate on the economic advantages of slavery
C) they supported Columbus and therefore reaped the rewards of his ventures
D) they established a chain of feitorias down the Atlantic coast supporting their position off Africa's west coast.