51. Which enzyme pair represents the key reciprocal regulatory point between glycolysis and gluconeogenesis? (A) Hexokinase and glucose‑6‑phosphatase (B) Pyruvate kinase and pyruvate carboxylase (C) Aldolase and triosephosphate isomerase (D) Phosphofructokinase‑1 and fructose‑1,6‑bisphosphatase