True Developmental Change Can Only Be Studied Through Longitudinal and Mixed

True developmental change can only be studied through longitudinal and mixed longitudinal designs because:

A) Motor development research involves the study of changes that occur in motor behavior over time.
B) Cross-sectional method yields only average age differences in groups across time, and not real individual changes across developmental time.
C) Longitudinal methods measure age-related changes in behavior.
D) These are the only two types of research designs.
E) Motor development research involves the study of changes that occur in motor behavior over time, cross-sectional method yields only average age differences in groups across time, and not real individual changes across developmental time, and longitudinal methods measure age-related changes in behavior.