Read the following passage and answer questions 48-50.       Epidemiologist Gerardo Heiss, and other researchers from the University of North Carolina, looked for signs that could be used to predict a person 's mortality rate. The researchers studied 2541 men, aged 40 to 69, over a 10-year span, Among the men who had cardiovascular disease at base line, those with high blood cholesterol levels (above 6.19 mmol per liter) had a higher risk of death from cardiovascular disease, including coronary heart disease, which was 3.45 times higher than that for men with desirable blood cholesterol levels. Cholesterol levels were also significant predictors of death from cardiovascular disease in men without preexisting cardiovascular disease, although at a lower level of absolute risk of death.
【題組】48. Based on contextual clues, what does an epidemiologist study?
(A)   how diseases are found. spread. and controlled in groups of people
(B)   how synthetic materials can be developed for medical applications
(C)   how characteristics are passed down to children by parents' genes
(D)   how diets and supplementary nutrients influence human health
(E)   the characteristics and effects of poisons and toxins