For Years, Sylvester the Cat Has Been Fed Canned Tuna

Multiple Choice

For years, Sylvester the cat has been fed canned tuna 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 unconditioned stimulus in this example?

A) the sound of the can opener
B) Sylvester's owner
C) food other than his
D) the tuna