In a school, there are four kid classes, with 21, 24, 17 and 28 students respectively. We choose one of the 90 students randomly. Let A denote the number of students in the class of the randomly selected student. One of the four teachers is also randomly selected. Let B denote the number of students in this teacher's class.

(a) Before explicitly calculating E[A] and E[B], which do you think is larger? Why?

(b) Find E[A] and E[B].