Pavlov Paired the Sound of a Metronome with Meat Powder

Pavlov paired the sound of a metronome with meat powder in a dog's mouth.As a result,the dog started salivating to the sound of the metronome.Later,Pavlov presented the metronome without the meat powder.Eventually,the dog quit salivating when it heard the metronome.This is an example of:

A) spontaneous recovery
B) operant extinction
C) extinction of a conditioned response (respondent extinction)
D) resurgence