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 )}