At the end of a semester, students' math grades are listed in a table which gives each student's ID number in the left column and the student's grade in the right column. Let $N$ represent the ID number and $G$ represent the grade of each student.

Which of the following statements regarding the relationship between student ID number and grade is true?
A. $G$ must be a function of $N$.
B. $N$ must be a function of $G$.
C. All of the above
D. None of the above