C 6 H 5 − CHO X = Y ≡ Z A − B = C ≡ D A − B = C ≡ D A - - - B - - - − C A - - - − − B - - - − − C − - - - − D A ⋅ ⋅ ⋅ B ⋅ ⋅ ⋅ ⋅ C A → B ← C A ?? B