Ants carry dead ants out of an anthill and dump them on a "trash" pile. If a chemical from a dead ant is applied to a live ant, other ants will carry it, kicking and struggling, from the anthill, until the substance wears off. Which of the following explains this behavior?
The ants have become imprinted on the chemical.
The ants are following a stimulus-response chain.
The ants can only learn by operant conditioning.
The ants are responding to a circadian clock.
The chemical is a sign stimulus for a fixed action pattern.