In a population-based cohort study, an entire community was interviewed regarding smoking habits and then followed for one year. Upon ascertainment of all lung cancer deaths, the investigator obtained the following data: Number of individuals Lung cancer deaths Smokers 24,500 15 Non-smokers 10,500 2 Calculate the risk difference. Group of answer choices 0.0001 0.0002 0.0003 0.0004