Why was Ader and Cohen's (1975) study on classical conditioning of the immune system so important?
A) It marked the beginning of the field of psychoneuroimmunology research.
B) It was the first time psychological research with chimpanzees had been conducted.
C) It showed that adherence could happen unintentionally.
D) It provided further evidence that HIV-positive patients can be either asymptomatic or symptomatic.