diff --git a/exam/ex.tex b/exam/ex.tex
index ab53422..bf71734 100644
--- a/exam/ex.tex
+++ b/exam/ex.tex
@@ -272,7 +272,9 @@
 
     \item \TODO
 
-    \item \TODO
+    \item If $G_{ch}=\phi_{ch}(G)$ and $G'=\psi(G)$, it follows that
+      $G=\phi_{ch}^{-1}(G_{ch})$ and therefore $G'=\psi(\phi_{ch}^{-1}(G_{ch}))$
+      so the verifier will always accept.
 
     \item \TODO