You decide that you are going to condition your dog to salivate to the sound of a metronome. You sound the metronome and then several minutes later you give the dog a biscuit. You do this several times but no conditioning seems to occur

This is probably because
a. the metronome was not a distinctive sound.
b. the metronome should have been sounded after the animal ate the biscuit.
c. you should have had an even longer interval between the metronome and the biscuit.
d. the biscuit was given too long after the sound of the metronome.
e. the metronome was not loud enough to be heard by the dog.

d. the biscuit was given too long after the sound of the metronome.