One of the central ideas in the above passage is: People often give out unconscious cues. What is the other central idea?
A. Researchers can prevent the Clever Hans effect from interfering with scientific experiments.
B. Von Osten did not realize that he was providing signals to his horse.
C. Answering questions via computer is one way to prevent the Clever Hans effect.
D. A parrot could be asked to identify colors in a scientific experiment.
