Missing a Test Question: A Case Study

I recently gave a test to my Geometry Concepts students which included the following:
Write a counterexample for the following: There is no number between 2 and 3.
First off, the only right answer students put was:
2.5
No 2.1, no 2 1/2, no 2.67. Only 2.5, in decimal. I don’t know if there’s a conclusion to be made [...]