44. In the citric acid cycle, citrate is isomerized to isocitrate by aconitase. H2O was first abstracted from citrate, and then rehydrated with H- and HO–adding back in opposite positions to produce isocitrate. This reaction catalyzed by aconitase is quite stereospecific. As citrate has a total of four chemically equivalent hydrogens (labeled as H1~H4 in the figure), only one of the four hydrogens, the pro-R H atom of the pro-R arm of citrate, is abstracted during the reaction of theH2O abstraction by aconitase.Which one of the hydrogens in the citrate molecule shown in the figure is the pro-R H atom of the pro-R arm?(A) H1 (B) H2 (C) H3 (D)H4 (E) None of them.