The correct answer is Both A and B are true. Key Points Thermal decomposition: Thermal decomposition is a chemical reaction where heat is a reactant. Since heat is a reactant, these reactions are endothermic meaning that the reaction requires thermal energy to break the chemical bonds in the molecule. Calcium carbonate, CaCO3, will decompose into carbon dioxide, CO2, and calcium oxide, CaO, when heated above 900 degrees Celcius at a pressure of the atmosphere. Additional Information Reversible reaction: ​A reversible reaction is a reaction in which the conversion of reactants to products and the conversion of products to reactants occur simultaneously. A and B can react to form C and D or, in the reverse reaction, C and D can react to form A and B. This is distinct from a reversible process in thermodynamics. The forward reaction N 2 ​ (g)+3H 2 ​ (g)2NH 3 ​ (g) (the production of ammonia) is exothermic. According to Le Chatelier's Principle, this will be favoured if you lower the temperature, a very low temperature will cause a reaction to occur very slowly and hence, not efficiently. Therefore, 400 - 450°C is a compromise temperature producing a high proportion of ammonia in the equilibrium mixture.