Statement A: The photochemical reactions H2+Cl2→2HCl and H2+Br2→2HBr have equal quantum efficiencies.Statement B: Both the reactions proceed by different mechanism.

H2+Cl2→2HCl has much higher quantum efficiency than  H2+Br2→2HBr. The first step of secondary process of H2+Cl2→2HCl is exothermic while the same for H2+Br2→2HBr is endothermic.