Graph A shows function f of x equals 3 multiplied by 3 to the power of x. Graph B shows function f of x equals 9 to the power of x. Graph C shows function f of x equals 3 multiplied by 3 to the power of negative x. Graph D shows function f of x equals 9 to the power of negative x. Which graph best shows the function f(x) = 3(3)−x to represent the rate at which a radioactive substance decays?