Why does the author mention "Vasco da Gama' Voyage around southern Africa to India" in the passage? A. to indicate how the Portuguese came to challenge Venetian dominance of trade with the East B. to explain why political troubles resulted in the closing of the usual routes to India C. to prove that Venetians could not sail round ships as efficiently as sailors from other countries did D. to show that Venetian reliance on round ships rather than galleys proved to be weakness