One of the Early Terms for Behaviourism Is 'S-R' Psychology

One of the early terms for behaviourism is 'S-R' psychology. What was the main theory of S-R psychology?

A) Everything we do is based on the gradual buildup of associations between stimuli and responses.
B) All of our voluntary behaviours are merely responses to classically conditioned stimuli.
C) Responses are only meaningful if you can identify the triggering stimuli.
D) Measurement of stimuli and responses is impossible, so we must build theoretical models to explain associations.