Some students off campus -> Are in choir -> All choir members are music majors

I got the wrong answer, but I don't understand why the correct choice is correct. As is stated in the title the inference says "some" choir members live off campus, and therefore the professor must be wrong to say that students living off campus are not music majors, because "some" is going to mean that at least one choir member who is a music major lives off campus, thereby disproving the generalization. I know I'm making some kind of mistake, but I keep coming to this conclusion...