I'm worried about NVR in general as I feel the subject is fraught with ambiguity.
Now, I'm not saying that absolutely all of the questions in any paper are, but just that there are some questions (~4%) in most papers for which a completely logical explanation can be given as to why some other choice can be the answer too.
I have found this in some VR papers as well (eg: those of of the type which ask to pick "odd one out").
My problem at the moment is trying to convince my DC that answering "(a) or (d)" will not fetch him any marks inspite of having absolutely reasonable explanations for both! (yes.. he does seem to enjoy winding me up!)