Ghosts?

In my opinion, I would like to see the other dead three liars as Cadence's imagination.  As we all know, ghost as a concept which is not yet proved. We don't know if there are really ghosts exsisted in this world. Also if we talk about the background and the situation of Cadence, we would like to consider this as an imagination. Because, she got brain injured and took madecine with the influence similar to drugs. This also can physically make her get imagination easily. And also we know how close Cadence and the three liars are. So I believe this is perfectly her own imagination.