The video explanation was a bit murky here. I'm not so sure why the answer choice is C. I have a good theory on why people likely mistakenly pick C even though it is the right a/c. There are some issues I have with it.
1. its trying to say that ...
... on 18. I set my diagram up to have my shelves ... other questions right and my diagram is just wrong for this ... that I set up my diagram? My diagram is ok? I'm ... the 3rd shelf with my diagram.