Which of the following ordered pairs define a function?
a. {(1,2), (2,4), (5,9), (2.3}}
c. {(1,2), (2,3), (3,4), (3,5)}
b. {[0.1), (1.1), (2,3), (4.2)}
d. {(1,1), (2,1), (2,3), (3,5)}