Research on Aging Is Particularly Difficult for All of the Following

Research on aging is particularly difficult for all of the following reasons EXCEPT:

A) psychologists study age spans as large as sixty years.
B) most research on aging uses cross-sectional designs which can result in cohort effects.
C) it is difficult to separate age-related declines in performance from confounds like disease,drug use,and lack of recent use of skills.
D) participants have a vested interest in the research and remain committed to participation in longitudinal studies.