Researchers who study adolescent health were excited to read the announcement that the National Institutes of Health released a dataset from the Adolescent Brain Cognitive Development Study, considered the largest long-term study of brain development. When scientists use this database to study the various factors that influence adolescent brain, cognitive, social, and emotional development, what type of research method are they doing?
a) Longitudinal study
b) Cross-sectional study
c) Experimental study
d) Observational study