7. The two moles of COz produced in the first turn of the citric acid cycle have their origin in the
(A) carboxyl and methylene carbons of oxaloacetate.
(B) carboxyl group of acetate and a carboxyI group of oxaloacetate.
(C) carboxyI group of acetate and the keto group of oxaloacetate.
(D) two carboxyI groups derived from oxaloacetate.
(E) two carbon atoms of acetate.