The graphs below show four functions: 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?