As the number of boxes sold increases by one unit, the total amount of money earned increases by $18
1) Considering that y=18x denotes a proportional relationship. We can write the following
x = boxes
y= $
y=18x See the behavior in the table
x (boxes) | y ($)
1 18
2 36
3 54
4 72
(...) (...)
2) Examining the options we can state that as the number of boxes increases by 1 unit, the total amount of money earned increases by $ 18.
3) Hence, the answer is C