Which of the following can be modeled by an exponential function?

	Two trains are 275 miles away from each other. Train A is traveling towards Train B at 35 miles per hour while Train B is traveling towards Train A at 50 miles per hour.
	The amount of an isotope that remains over time given that we start with 800 mg and the isotope has a half-life of 36 hours.
	The amount of money in your piggy bank over the course of four months given that your aunt gives you $10 every week and you save the full amount.
	An online streaming service charges a flat fee of $5 per month and $1.50 for each movie rented.