Which choice could NOT be modeled by an exponential function? the amount of money, y, in an account after x years earning 4% interest compounded annually

the monthly cost, y, to use a cell phone for x minutes at a rate of 4 cents per minute

the amount, y, of radioactive material remaining after x years when decay occurs at a rate of 30% each year

The management of a supermarket wants to adopt a new promotional policy of giving a free gift to every customer who spends more than a certain amount per visit at this supermarket. the expectation of the management is that after this promotional policy is advertised, the expenditures for all customers at this supermarket will be normally distributed with a mean of $95 and a standard deviation of $20. if the management wants to give free gifts to at most 10% of the customers, what should the amount be above which a customer would receive a free gift?