"Nation” as a free root can function alone in a sentence while "diet" is a ______, which can not function alone grammatically and has to combine with other morphemes to create new words.

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.