Which of the relations given by the following sets of ordered pairs is a function? { ( 2 , - 8 ) , ( 1 , - 4 ) , ( 0 , 0 ) , ( 1 , 4 ) , ( 2 , 8 ) } { ( 3 , - 3 ) , ( 3 , - 1 ) , ( 3 , 1 ) , ( 3 , 3 ) , ( 3 , 5 ) } { ( 1 , 2 ) , ( 2 , 3 ) , ( 3 , 4 ) , ( 5 , 6 ) , ( 2 , 1 ) } { ( - 2 , 5 ) , ( 7 , 5 ) , ( - 4 , 0 ) , ( 3 , 0 ) , ( 1 , - 6 ) }