The correct answers are A) Southerners were able to maintain control of the presidency because of the number of electors from each state, and C) Southerners were able to pass key pieces of legislation through Congress.
During the 1787 Constitutional Convention, the Three-Fifths Compromise was the agreement reached by the delegates of the states that counted every slave in the United States territory as three-fifths of a person for porpuses of representation and taxation. This agreement gave the southern states more power. The decision also helped spread slaves in the south and displaced Native American Indians tribes.
So it is correct to say that The Three-Fifths Compromise changed politics in the United States before the Civil War in that Southerners were able to maintain control of the presidency because of the number of electors from each state, and Southerners were able to pass key pieces of legislation through Congress.