PLEASE HELP! Which of the following situations can be modeled with an exponential function?

a.) The number of fish in a tank increases by five per month.
b.) The number of people in a city increases by 100 every year.
c.) The value of a car at the end of a year is 8 percent less than the value at the beginning of the year.
d.) The value of a phone drops $100 per year.