You are answering a USMLE-style medical question. Question: A group of researchers from Italy conducted a case register study that included all patients from a city who had an International Classification of Diseases diagnosis and contacts with specialist psychiatric services over a 10-year period. Mortality was studied in relation to age, sex, diagnosis, care pattern, and registration interval. Standardized mortality ratios (SMRs) were calculated, with the overall SMR being 1.63 (lower when compared with studies that looked at hospitalized individuals). Men and those in younger age groups showed higher mortality rates (SMRs of 2.24 and 8.82, respectively), and mortality was also higher in the first year following registration (SMR = 2.32). Higher mortality was also found in patients with a diagnosis of alcohol and drug dependence (SMR = 3.87). The authors concluded that the overall mortality of psychiatric patients managed in a community-based setting was higher than expected; however, it was still lower than the mortality described in other psychiatric settings. The primary measure used in this study can be defined as which of the following? Options: A) The total number of deaths divided by the mid-year population B) The number of deaths in a specific age group divided by a mid-year population in that age group C) The observed number of deaths divided by the expected number of deaths D) The number of deaths from a certain disease in 1 year divided by the total number of deaths in 1 year Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.