Nandc.7z

The "piece" you are looking for depends on your current task:

If you are trying to open the file and it is locked, the most common password for such educational or "puzzle" archives is often nandc , password , or 1234 . NandC.7z

If this is for a coding assignment, the "piece" refers to the symbols 'X' and 'O' . The "piece" you are looking for depends on