Which of the contexts below could not be modeled by an exponential function?

A certain population of 16 aggressive zombies quintuples every day.

A taxi charges a flat fee of $2.00 for pick-up, then an additional fee of 4.75 per mile.

A coin collection appreciates at a rate of 5.2% per year.

A car depreciates at a rate of 4.9% per year.