The Community Health Center provides early childhood checkups and a vaccination clinic for children. It pays annual rent of $25,000 and has a permanent staff consisting of a nurse practitioner and a secretary who are paid a total of $145,000 per year. The center hires physicians on a per diem basis to provide exams and uses $15 worth of supplies for each exam not including vaccines. Appointments for the vaccination clinic are scheduled in advance, and the Center needs one per diem doctor for every 100 children they have scheduled on any given day. It also spends $7.50 per child for the vaccines used in its vaccination program. Which of the Center's expenses are semi-fixed (or step-fixed)?