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Which of the following relations is a function?
a. {(3,0), (6,1), (6,2), (3,3)}
b. {(1,5), (2,6), (-3,7), (1,8)}
c. {(2,2), (4,2), (-2,1), (8,0)}
d. {(3,1), (-5,7), (3,10), (1,1)}​