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 HsㅡC-HA hydrogens, the pro-R H atom of the pro-R arm of citrate, is abstracted during the reaction of the H2O 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.