Passionate kissing naturally causes Jon to be sexually aroused. Whenever Jon's partner wears red lipstick, they kiss him passionately. Eventually, Jon gets sexually aroused whenever he sees red lipstick. Jon then realizes that before putting on red lipstick, his partner always takes long showers. Eventually, Jon gets sexually aroused whenever his partner takes long showers. This best describes:
A. unconditioned stimulus
B. neutral stimulus
C. conditioned stimulus
D. higher-order conditioning