Tanvir is a graduate student who is studying identity formation. He selects a group of 5-year-olds, a group of 10-years-olds, and a group of 15-year-olds, and a group of 15-year-olds, and interviews each group asking them what they plan to be when they finish school. In this example, Tanvir is using:

A. a multi-factorial research design
B. a longitudinal research design
C. a cross-sectional research design
D. a nested condition research design