More fun with auto-fill, never seen 21-D before. Sure most will not get 3-D off the bat, but it's a sound answer. Should it have a question mark? No, it's not wordplay.
Been making puzzles for a year or two. Just trying this site.out for feedback and such. This theme just kind of happened organically as I put words into the long spaces ....then added more short entries as I finished the puzzle.