Before polling students in the Scion of Business, a researcher divides all the current students into groups based on their class standing (freshman, sophomores, etc). Then, she randomly draws a sample of 50 students from each of these groups to create a representative sample of the entire student body in the school. Which of the following sampling methods is the researcher practicing?
A. Simple random sampling
B. Systematic random sampling
C. Stratified random sampling
D. Cluster sampling
E. Snowball sampling