Several randomized trials have shown that lowering blood pressure can reduce the risk of cardiovascular mortality. In addition, findings of observational studies have indicated that elevated serum phosphorus can increase the risk of cardiovascular mortality through vascular injury and calcification. In 2012, a group of investigators sought to examine the joint effect of hypertension and elevated serum phosphorus on cardiovascular mortality. Information on blood pressure and serum phosphorus levels among individuals participating in the Third National Health and Nutritional Examination Survey 1988-1994 (NHANES III) was used. The records of NHANES III participants were linked with the National Death Index to ascertain their vital status through December 31, 2006. Source: Vart P, Nigatu YT, Jaglan A, et al. Joint effect of hypertension and elevated serum phosphorus on the risk of mortality in National Health and Nutrition Examination Survey-III. JAHA 2015; 4:e001706. a. What specific epidemiologic study design was used by these investigators to answer their research question? (Include how and when exposure and outcome were measured, if the study was experimental or observational, if the study was prospective or retrospective, as well as the name of the specific study design.) Explain your answer. b. The table below presents the risk of cardiovascular mortality for different study groups relative to that of individuals who had neither hypertension nor elevated serum phosphorous. Based on this information, is there evidence of multiplicative interaction between blood pressure and serum phosphorus levels in their effect on cardiovascular mortality? Show your calculations and explain your answer. Group Relative risk No Hypertension AND No elevated serum phosphorus Hypertension AND 1 -No elevated serum phosphorus 1.80 -Elevated serum phosphorus 2.80 No Hypertension AND -Elevated serum phosphorus 1.01