Lily, a child psychologist, is investigating variables potentially associated with bullying among school-aged children. In 2018 she selected three samples of children: a group of 9-year-olds, a group of 11-year-olds, and a group of 13-year-olds. She interviewed the children and asked them to complete several questionnaires. Lily plans to repeat the procedure with the same groups of children in 2021 and in 2024. Lily's study exemplifies a _____ research design.

a. longitudinal
b. cross-sectional
c. cross-sequential
d. factorial