Christopher Columbus is the explorer who is credited for discovering America. Of course, there were already people living in America at the time who we call Native Americans. There even was a European, Leif Ericsson, who had been to the Americas before. However, it was Columbus' voyage that started the exploration and colonization of the Americas.
The answer to this question I think is 3 Laos
Constantinople was able to survive due to its <em>strategic</em> location and <em>sofisticated</em> system of fortifications.
Constantinople was able to survive as the <em>central</em> hub of the European culture throughout the early 1000s due to its strategic location, the city was located in a strait between the Black and Mediterranean Seas, between Europe and Asia, which allowed the city to be prevented from many invaders, from Hunes to Arabs.
In addition, the city counted with a sophisticated system of fortifications which made impossible any attempt of siege until the appearance of first developments on modern artillery in the 15th Century, and the city was well communicated with the most important nations in Eastern and Western Europe by sea (i.e. Italy, Carolingian Empire, Kiev' Rus), of which the city may receive reinforcements in desperate situations.
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