Answer:
C
Step-by-step explanation:
A function is where each input has a single output.
(A) There can be a different amount of students in a class (e.g. 21 or 22) that have the same class length
(B) There can be multiple things that Mike can order with $1
(D) There can be multiple students have the same GPA
(C) There is one button for each snack item
