The passage asserts which of the following about the exposures involved in the "research on olfactory adaptation?" A. The exposures are of long enough duration for researchers to investigate many aspects of olfactory adaptation. B. The exposures have rarely consisted of reexposures following extended absences from the odorous environment. C. The exposures are intended to reproduce the relatively transient olfactory changes typical of exposures to odors in natural environments. D. Those exposures of relatively short duration are often insufficient to produce the phenomenon of receptor fatigue in study subjects. E. Those exposures lasting several hours produce reductions in receptors in the olfactory epithelium that are similar to the reductions caused by prolonged odor stimulation.