A medical study involving senior citizens was designed to determine the relationship between the amount of salt in the diet and blood pressure. What are the independent and dependent variables for this study? A. The independent variable is blood pressure, and the dependent variable is amount of salt in the diet. B. The independent variable is the age of the senior citizens, and the dependent variable is blood pressure. C. The independent variable is blood pressure, and the dependent variable is the age of the senior citizens. D. The independent variable is the amount of salt in the diet, and the dependent variable is blood pressure.