An example of N. A. is the continent that contains Canada and the United States.
An example of N/A is what you enter on a form when you are asked your date of marriage and you have never been married.
An example of Na is what is found in salt.
"Na, yor wrang."
"Na, ye cannet watch telly"
"Divn't yee like milk?" "'Na" (i.e., "No, I don't like milk.")