I get that the answer to this should be C, but can it also not be D? Because how do we know that there are no other categories of students besides X&Y?
If someone is a student,
He either goes to College C, and then they will belong to category x, or he goes to some other college, in which case they will belong to category y.
No of students = Students who go to college c (x) + Students who go to other colleges (y)
There is no other categories for a student.