which situation could be modeled by a linear function

a band account that grows at a rate of 5% per year, compounded annually

a population of bacteria that doubles every 4.5 hours

the cost of cell phone service that charges base amount plus 20 cents per minute

the concentration of medicine in a persons body that decays by a factor of one-third every hour

The cost of cell phone service that charges a base amount plus 20 cents per minute could be modeled by a linear function.