Identify the correct statement regarding enzymes.

• A
  Enzymes are specific biological catalysts that cannot be poisoned.
• B
  Enzymes are normally heterogeneous catalysts that are very specific in their action.
• C
  Enzymes are specific biological catalysts that can normally function at very high temperature $(T \sim 1000 \ K)$.
• D
  Enzymes are specific biological catalysts that possess well-defined active sites.