Research Showing That Animals Classically Conditioned to Salivate to a Tone

Multiple Choice

Research showing that animals classically conditioned to salivate to a tone that is always paired with food make a stronger conditioned response than animals conditioned to salivate to a tone that is sometimes paired with food. This is evidence that which factor is most important in classical conditioning?

A) The time between the CS and US
B) The similarity between the CS and US
C) The extent to which the CS predicts the US
D) All of the above