Data Sufficiency

could someone please explain this?

Very easy question

We are told that

1.0 there are 800 students
2.0 250 live on campus and are more than 20 years old

Lets look at the statements separately

Statement I : 640 are more than 20 yrs , so from this we only know that 160 are less than 20 and 250 of them live on campus

Statement II : 60 are 20 or less and live off campus . That's it .it doesn't give anything more

So A , B , and D are out

Combine Statement I , II and the givens we get the answer so its C

Thank you