Factory began dumping toxic waste materials into a countryside lake. Before the dumping began, the estimated population of striped bass in the lake was 40,000. Since the dumping of toxic waste materials began, the population has decreased by one-half every month. Determine the equation that represents this situation and use it to decide which of the following graphs represents the population of striped bass, f(x), x months after the factory began dumping toxic waste materials into the lake.