Which of the Following Is Not a Function

Multiple Choice

Which of the following is not a function?

A) {(A, 1) , (B, 2) , (C, 3) }
B) {(A, 1) , (B, 1) , (C, 3) }
C) {(A, 1) , (B, 2) , (B, 3) , (C, 1) , (C, 2) }
D) {(A, 2) , (B, 2) , (C, 2) }