Line Number for all non-blank lines?

Kind of amused here, I'm trying to pass the [prereqs](https://classroom.udacity.com/courses/ud219/lessons/972928869/concepts/9977987120923) to get into the [Udacity Course on High Performance Computer Architecture by Georgia Institute of Technology](https://www.udacity.com/course/high-performance-computer-architecture--ud007) For the life of me I can't figure out what they want, Question reads, > To list the contents of a file, with line numbers for non-blank line shown use the command with an option: ________ ___ filename. I've tried nl. It didn't work. Most bizarre question ever.