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 $l$ 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 $t-s$ ? 
选项为:A:$$-18.5$$  B:$$-13.5$$  C:$$0$$  D:$$13.5$$  E:$$18.5$$  请回答其中一个选项.