A --> B
(A)+ = {A,B} i.e. A is the key and therefore for any value of A, we can determine B.
B --> A
(B)+ = {B,A} i.e. B is the key and therefore for any value of B, we can determine A.
Therefore for every unique value of A there is a unique value of B and for every unique value of B there is a unique value of A. It can be possible only in One to One relation.