Bechterev Had Human Subjects Place Their Fingers on a Shock

Bechterev had human subjects place their fingers on a shock plate, signaled with a warning CS, and then delivered a brief shock.He noticed that after a few trials, subjects were lifting their fingers from the plate in response to the CS, thus reducing the shock US.This experiment is best characterized as an investigation of

A) aversive classical conditioning mechanisms.
B) passive avoidance.
C) active avoidance.
D) omission conditioning.