A school has 100 students and 5 teachers. In the first period, each student is taking one class, and each teacher is teaching one class. The enrollments in the classes are 50,20,20,5, and 5. Let t be the average value obtained if a teacher is picked at random and the number of students in their class is noted. Let s be the average value obtained if a student was picked at random and the number of students in their class, including the student, is noted. What is t−s?<spanclass=′latex−bold′>(A)</span>−18.5<spanclass=′latex−bold′>(B)</span>−13.5<spanclass=′latex−bold′>(C)</span>0<spanclass=′latex−bold′>(D)</span>13.5<spanclass=′latex−bold′>(E)</span>18.5