Question

Which of the relations given by the following sets of ordered pairs is a function?

[] {(1,2),(-4,8),(-3,5),(1,-2),(7,12)}
[] {(6,1),(-4,1),(3,5),(2,4),(-7,2)}
[] {(5,4),(4,3),(3,2),(4,5),(2,1)}
[] {(5,4),(5,6),(5,8),(5,10),(5,12)}