Carson used to enjoy lime sherbet, so when he was in Mexico, he tried frozen lime margaritas. After his fourth margarita, Carson became extremely ill. Now he finds that even the sight of lime sherbet in a bowl can make him feel queasy. In this example, the unconditioned stimulus is:________.
a. the lime margaritas that Carson consumed.
b. the illness that followed the fourth margarita.
c. the sight of lime sherbet.
d. the queasiness that Carson feels when he sees lime sherbet.