Dr. Morales studies morality in children. In her latest study she followed and assessed 7-year-olds, 8-year-olds, and 9-year-olds for a period of time. The 7-year-olds were tested at ages 7, 8, and 9; the 8-year-olds at 8, 9, and 10; and the 9-year-olds at ages 9, 10, and 11. This is an example of a __________ research design. A. long-term B. survey C. cross-sectional D. cross-sequential