Which of the following is not a one-to-one function?

a. {(1,2),(2,3),(3,4),(4,5)}
b. {(1,1),(2,2),(3,3),(4,4)}
c. {(2,1),(3,2),(4,3),(5,4)}
d. {(1,3),(2,1),(4,3),(4,5)} ​