What can be known about this situation? Chemicals A and B form an endothermic reaction, and chemicals C and D form an exothermic reaction. Chemicals A and B form an exothermic reaction, and chemicals C and D form an endothermic reaction. Chemicals A and C form an endothermic reaction, and chemicals B and D form an exothermic reaction. Chemicals A and B form an exothermic reaction, and chemicals B and D form an endothermic reaction.