Answer:
4. A gym charges a one-time membership fee plus a monthly fee of $19.99.
Step-by-step explanation:
Linear functions increase or decrease by the same amount at each interval.
For example, as the x value increases by 1, the y value increases by 5.
So, option 4 is correct, because the fee is the same each month.
The other answer choices are not linear because they increase or decrease by percentage values.