Which species can act as an oxidizing agent but not as a reducing agent?
(A) $\ce{Cl2}$
(B) $\ce{Cl-}$
(C) $\ce{ClO2-}$
(D) $\ce{ClO4-}$
I thought the answer was A because if $\ce{Cl}$ is reduced, it becomes very stable with a full octet and therefore cannot be the reducing agent; however, according to the 2007 Chemistry Olympiad answer key, the answer is D. Why is that?