The answer is Word Count, this is correct because the options "Word Number" and "Word Length" do not make much sense when trying to calculate the total number of words in a piece of writing. And, not all pieces of writing are stories, so you can cross of "Story Length" as an option too. Leaving you with "Word Count", which can be the accumulative number of words in a piece of writing.
You must walk in the halls, not run, you can get seriously hurt. You can trip and scar up your knees, you can tumble and fall, and hit your behind. hope this helps :)
