Which one is a unit vector?

• A. ??3 i^ + ??3 j^ + ??3 k^
• B. 1/??3 i^ + 1/??3 j^ +1/ ??3 k^
• C. ??3 / 3 i^ + ??3 / 3 j^ + ??3 / 3 k^
• D. Both b and c are correct