Hi, good question! This stems from the fact that unless statements are (in my opinion) the most frustrating conditional to diagram. My favorite approach is to replace the word unless with "if not" or "if it is not the case."
Here, we can rephrase the initial statement to "nursing schools will not get more students if they do not solve problem 1 and 2.
This is way way easier to diagram: if not solve 1 and 2, then not more students.
And it's contrapositive If more students, then we must have solved the two problems.
To answer your question more directly, I don't think we need to switch it out because we didn't actually take the contrapositive here. More specifically, we need to solve both of the problems- D is telling us that we only must have solved one of them.
