I chalked it up to high school biology knowledge. I'd read up on what the largest single-celled organisms are like for both functions and size. Using the term "worm", even if small, indicates a certain level of complexity that probably precludes the possibility of it being a single-celled organism. That said, I still had the same thought in the back of my mind that single-celled organisms could have been responsible until I saw that none of the answer choices really addressed that.