Dr. Anders wants to investigate how people of different ages communicate via the Internet, so she does an experiment with three groups of three different ages: 18–21, 47–50, and 75–78. Concurrently comparing how people of different ages behave is an example of which of the following kinds of research?

a. Case study

b. Longitudinal

c. Cross-sectional

d. Factor analysis

e. Qualitative research