Which relation is a function?
A) {(2,3). (5,8).(-4,-1). (9.7), (2.1)}
B) {(2,3). (5,8).(-4-1). (9.7). (5,7)}
C) {(2,3). (5.8).(-4,-1). (9.7).(-2.1)}
D) {(2,3). (5.8).(-4,-1). (5,7), (-2,1)}