Which Statement About Additional Forms of Pain Control Is True

Which statement about additional forms of pain control is true?

A) Acupuncture-induced analgesia is blocked by µ-receptor antagonists.
B) Although inhibiting the two peptidases that degrade enkephalin produces significant analgesia, addiction potential is a concern.
C) An initial gene therapy trial used a virus engineered to contain the gene coding for human proenkephalin.
D) Although NOP-R agonists reduce pain, their use is limited by side effects such as constipation and respiratory depression.