(3 points) Each student at some college has a mathematics requirement M (to take at least one mathematics course) and a science requirement S (to take at least one science course). A poll of 150 sophomore students shows that: 60 completed M, 45 completed S, and 25 completed both M and S. Find the number of students who have completed: (a) (1 point) At least one of the two requirements (b) (1 point) Exactly one of the two requirements (c) (1 point) Neither requirement.