Which of the following relations are functions?
A{(a,b),(b,a),(c,c),(e,d)}
B{(c,e),(c,d),(c,b)}
C{(b,b),(c,d),(d,c),(c,a)}
D{(a,b),(b,c),(c,d),(d,e)}
A{(a,b),(b,a),(c,c),(e,d)}
B{(c,e),(c,d),(c,b)}
C{(b,b),(c,d),(d,c),(c,a)}
D{(a,b),(b,c),(c,d),(d,e)}