Kathy's Heart Rate Increases Every Time She Goes Down a Certain

Multiple Choice

Kathy's heart rate increases every time she goes down a certain hallway at school. The boy that she likes has a locker in that hallway, and he often stands beside his locker and talks with friends. In the preceding scenario, what is most likely the unconditioned stimulus?

A) Kathy's heart rate
B) Kathy's attraction for the boy
C) The hallway where the boy's locker is located
D) The boy