The use of check and balances.
Jonas Salk<span> </span><span>was an American </span>medical researcher<span> and </span>virologist<span>. He discovered and developed one of the first successful </span>polio vaccines in the 1950's.
Hope I helped. :)
<span>Although the war persisted on the high seas and in other theaters, the Patriot victory at Yorktown ended fighting in the American colonies. Peace negotiations began in 1782, and on September 3, 1783, the Treaty of Paris was signed, formally recognizing the United States as a free and independent nation after eight years of war.
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The First Europeans. The first Europeans to arrive in North America -- at least the first for whom there is solid evidence -- were Norse, traveling west from Greenland, where Erik the Red had founded a settlement around the year 985.
