Athumbtack stuck in a bulletin board several feet away is less likely to affect your behavior than a thumbtack placed on the chair on which you are sitting. even though both examples involve a thumbtack, the second is more likely to qualify as a

Answer – Stimulus

Unlike a thumbtack stuck in a bulletin board several feet away, one that is placed on the chair on which you are sitting is more likely to qualify as a stimulus and can therefore elicit a sensory or behavioral response.

i would say b. research typically uses a variety of sources