For Years Sylvester the Cat Has Been Fed Canned Tuna

Multiple Choice

For years Sylvester the Cat has been fed canned tuna fish that has been opened with an electric can opener. Now Sylvester races into the kitchen every time he hears the can opener. What is the conditioned stimulus in this example?

A) The tuna fish
B) Sylvester's owner
C) The sound of the can opener
D) Running to the kitchen