Classify the situation as stratified random sampling, systematic random sampling, or cluster sampling. A college decided to survey its students to see how they felt about using a computer in all classes, and how much extra tuition they would be willing to pay for the service. The college determined that of its 10,000 students, 500 surveys would be sufficient to achieve satisfactory results. The students attending in that semester were sorted according to their student number, and every 20th student on the list was selected for the survey. The situation is classified as __________sampling.