The argument in the passage leads most naturally to which of the following conclusions A: A low output of urinary kallikrein is a likely cause of high blood pressure in children. B: The kallikrein-kinin system plays an important role in the regulation of blood pressure. C: Essential hypertension may have biochemical precursors which may be useful predictors in children. D: The failure of the body to produce sufficient amounts of kinins is the cause of essential hypertension.