You are answering a USMLE-style medical question. Question: A group of researchers wants to evaluate how often the human immunodeficiency virus (HIV) is related to high-risk behaviors in female sex workers from their country. An additional aim is to evaluate the association between HIV acquisition and certain sociodemographic factors. The researchers collect data through interviewer-administered questionnaires (for behavioral and sociodemographic data) as well as through clinical and serological evaluation/screening methods for HIV and other sexually transmitted infections. Which of the following could be the main outcome measure of their study? Options: A) Incidence B) Prevalence C) Hazard rate D) Attributable risk Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.