Which of the following is NOT mentioned in paragraph 2 as a problem in using the technique of direct observation? A. It is impossible to be certain of the actual cause of an infant's response. B. Infants' responses, which occur quickly and diffusely, are often difficult to measure. C. Infants do not respond well to stimuli presented in an unnatural laboratory setting. D. It may be difficult for observers to agree on the presence or the degree of a response.