43. In the citric acid cycle, succinate is dehydrogenated by succinate dehydrogenase, which yields fumarate. The dehydrogenation of two of the four hydrogens in succinate (labeled as H1~H4 in the figure shown below) is stereospecific, with the pro-S hydrogen removed from one carbon atom and the pro-R hydrogen removed from the other. Which of the following two hydrogens could be abstracted in the four hydrogens of the succinate molecule in the reaction catalyzed by succinate dehydrogenase? a. H1 and H2 b. H1 and H3 c. H1 and H4 d. H2 and H3 e. H2 and H4 f. H3 and H4 (A) b and e (B) c and d (C) a and f (D) Only c could occur. (E) Only d could occur.