Which relation is a function? : )
a. {( 1 , 2) , ( 2 , 3 ) , ( 3 , 4 ) , ( 4 , 5 )}
b. {( 1 , 2 ) , ( 1 , 3 ) , ( 1 , 4 ) , ( 1 , 5 )}
c. {( 1 , 2 ) , ( 2 , 3 ) , ( 3 , 4 ) , ( 2 , 5)}
d. {( 1 , 2 ) , ( 3 , 2 ) , ( 3 , 3 ) , ( 4 , 2 )}