Roland learned about classical conditioning in his psychology class, and he was eager to use it to ...

Roland learned about classical conditioning in his psychology class, and he was eager to use it to train his dog. He presented a tasty morsel, rang a bell, then watched the animal drool as it ate. He repeated this multiple times: food, bell, response. Then he rang the bell all by itselfand nothing happened. No drooling, no response, just an indifferent look from a bored dog. Why had no learning taken place?
a)  The CS needs to precede the US in order for conditioning to take place.
b)  The US and the CR need to be established separately before being paired together.
c)  The US and the UR need to be established separately before being paired together.
d)  Roland was actually extinguishing a UR using his technique.