In Pavlov's Studies of Classical Conditioning of a Dog's Salivary

In Pavlov's studies of classical conditioning of a dog's salivary responses, spontaneous recovery occurred

A) during acquisition, when the CS was first paired with the US.
B) during extinction, when the CS was first presented by itself.
C) when the CS was reintroduced following extinction of the CR and a rest period.
D) during discrimination training, when several conditioned stimuli were introduced.