Dr. Sardonicus is studying the effects of aging on memory. She gives an initial test of memory to a group of 20-year-olds and a group of 40-year-olds. Then she follows both groups of participants for a period of five years, regularly giving them memory tests during that time. Which research design is Dr. Sardonicus using?

• A. Longitudinal
• B. Cross-sequential
• C. Cross-sectional
• D. Genetic-potential