A- (0,0) , (1,1) , (2,2) , (3,3) ------------- A FUNCTION
B- (1,0) , (2,0) , (3,0) , (4,0) ------------- A FUNCTION
C- (0,1) , (1,2) , (2,3) , (3,4) ...................NOT A FUNCTION
D- (0,0) , (0,1) , (1,2) , (1,3) .................. NOT A FUNCTION