Pavlov's dogs exhibited stimulus discrimination when they _______.
◦ did not demonstrate a conditioned response upon hearing ticking sounds similar to the metronome.
◦ stopped experiencing the conditioned response in the absence of the unconditioned stimulus.
◦ salivated at the sound of the metronome after a few weeks of not hearing it.
◦ initially began salivating to the metronome when presented with food.