分枝定界法在需要分枝时必须满足: 一是分枝后的各子问题必须容易求解; 二是各子问题解的集合必须覆盖原问题的解；
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Which three of these things is true of George Washington?
A. the first President of the United States
B. the author of the Declaration of Independence
C. the Commanding General of the Continental Army during the Revolutionary War
D. he is one of four presidents whose heads are carved on Mount Rushmore
E. Instead of President he wanted to become America's first king
Which of these things was true of the Thirteen Colonies?
A. the colonies called New England were in the southern colonies
B. the 13 Colonies were found along the coast of the Atlantic Ocean
C. The colony called New York was originally started by the Dutch
D. The first colony to be started was called Virginia
E. 12 of the 13 colonies were started primarily to gain religious freedom
The US Constitution
A. created a unicameral Congress
B. stated that the Constitution could never be changed
C. created a government divided into three equal parts
D. is the highest authority in the US government
E. set up a system of state electors to choose the President
Slaves in America
A. were brought from Africa
B. mainly worked on southern plantations
C. mainly worked in northern factories
D. originally came seeking religious freedom
E. were freed through the Emancipation Proclamation
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