Mollie Loved to Eat Her Grandmother's Pecan Pie

Mollie loved to eat her grandmother's pecan pie.However, as her grandmother got older, she had difficulty picking out all the pecan shells, so some of the pieces got cooked into the pie.When Mollie would bite into a piece of pecan shell in the pie, she would grab her jaw in pain.After several visits eating "pecan-shell" pie, Mollie would grab her jaw before she even put a bite of pie in her mouth.The pain (of biting a pecan shell) is the

A) conditioned response (CR) .
B) conditioned stimulus (CS) .
C) unconditioned response (UR) .
D) unconditioned stimulus (US) .