7. Consider the following data on HIV seroconversion (from negative to positive) in a high- risk group of 650 subjects. Assume that HIV positivity is permanent, and there are no entries to or losses from the group of subjects. The study begins on January 1 2001 and as of this date, there are 18 participants who are found to be positive, on January 1 2002 there are 2 positives, and on January 1 2003 there are 5 positives. Calculate: a. The population at risk for HIV seroconversion on January 1 2001. b. The prevalence on January 15 2002. c. The cumulative incidence over this study period