Which of the following is a redox reaction? HBr (aq) + KOH (aq) rightarrow H_2 O (l) + KBr (aq) SO_3 (g) + H_2 O (l) rightarrow H_2 SO_4 (aq) HBr (aq) + Na_2 S (aq) rightarrow NaBr (aq) + H_2 S (g) NH_4^+ (aq) + 2 O_2 (g) rightarrow H_2 O (l) + NO_3^- (aq) + 2 H^+ (aq) None of the above