What evidence best illustrates that the perception of pain is influenced by experience?

What evidence best illustrates that the perception of pain is influenced by experience?
 
  a. When men think that other men are watching them, they report feeling less pain in response to an injury.
  b. Surgery patients whose family members were allowed to be by their side shortly after surgery reported less pain during recovery.
  c. Women who participated in childbirth classes reported less pain during the birthing process.
  d. Women reported less pain after receiving an electrical shock if they were allowed to look at pictures of their families.