Please help indicate the type of sampling used for this study.

Here is the summary of the study:

BACKGROUND: Find the dietary consumption of young adults who have Picky Eating Disorder (PED) and the relationship of PED and psychological outcomes. PROCEDURE METHODS: They went to a large Midwestern university. The sample was taken from undergraduate students who were enrolled in a social science course. The students that were interested in the survey could take the survey using an online survey website and were given extra credit.

is this systematic, stratified, cluster, and if not any of these what type is it?