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. The entire student body is assigned a number, and then a random number generator is used to select 100 of those numbers at random. The students matching those numbers are given the survey. What sampling method was used?