The experiment of Ivan Pavlov and his dogs is one of the most known and remarkable in the history of psychology. Thanks to this small accidental discovery, a psychological theory of learning began to be constructed. Thus, Pavlov's studies have helped us to understand associative learning through classical conditioning.
Pavlov said that humans and animals can have trained and learned responses. Through his experiments, classical conditioning was defined as the association of an initially neutral stimulus with a significant stimulus. Therefore, when the neutral stimulus is present, in the absence of the other stimulus, a similar response will arise that would occur if the stimulus were significant. This ability to associate stimuli, however disparate, helps us in many everyday situations.
To my knowledge, the correct answer among all of the given choices is C.) Steam. If I'm not mistaken, this was the time that more trains came into use.
I hope this is the answer you were looking for and that it helps!! :)
