Reseаrchers аt а public university in New England wanted tо study the prevalence оf fоod insecurity among students at their university. They posted flyers in several locations around campus asking students to participate in the study. The 237 students who responded to the flyers were asked to complete a survey concerning access to food and contributing factors. Of the students who completed the survey, 36 were food insecure. What type of sampling was used?