Which chlorine species can act as an oxidizing agent but not as a reducing agent? 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?
For a species to act as a reducing agent, it should have the ability to get oxidised. Any element in its highest oxidation state, cannot oxidise anymore, and can hence not act as a reducing agent. $\ce{ClO4-}$ has $\ce{Cl}$ in $+7$ oxidation state, and cannot have a higher oxidation state, and hence cannot act as a reducing agent.