Sorry, but there is not enough information in your post to really answer. One thing to note: on a lot of the quizzes, the order of the questions can change, so the question number is not determinative.
There is a question involving bounding boxes and IOU, but the problem people typically have is not to do with the boxes that don’t intersect. There are some other issues with understanding the effect that IOU actually has. Here’s a previous thread which I think is about the same quiz question. Please have a look and let us know if that doesn’t answer your question.