Which Statement Is Unsubstantiated

Which statement is unsubstantiated?

A) Intense endurance exercise is associated with the release of three types of endogenous opioids into the blood,of which endorphins are one type.
B) The hypothesis that released blood chemicals,such as endorphins,is responsible for positive changes in mood and affect is based largely upon logic and not cause and effect research.
C) Endogenously produced endorphins mimic the effects of injected morphine in rats.
D) The release of endorphins into the blood stream,during intense aerobic exercise,is responsible for the "runners high" and for positive changes in mood and affect.