A school has 5000 students. There are 1500 freshman, 1500 sophomores, 1000 juniors, and 1000 seniors. At 8:00 AM, all students are in class; there are 200 classes that each have 25 students.

The administration wants to give a survey asking the students about their favorite classes. They wish to poll 100 students.

They select 4 of the 8:00 AM classes at random. All students in the selected classes are given the survey.

What sampling method was used?