Concept:Temporary hardness of water is caused by dissolved calcium and magnesium hydrogencarbonates, which can be removed by boiling.Explanation:When rainwater dissolves carbon dioxide, it reacts with calcium carbonate and magnesium carbonate in the ground to form soluble hydrogencarbonates.These hydrogencarbonates are Ca(HCO3)2 (calcium hydrogencarbonate) and Mg(HCO3)2 (magnesium hydrogencarbonate).Temporary hardness is called "temporary" because boiling decomposes these salts, removing the hardness.For example:Ca(HCO3)2ΔCaCO3↓+H2O+CO2The insoluble carbonates precipitate, so the water becomes soft.Among the options, only option A contains exactly these two hydrogencarbonate salts.Answer:A. Ca(HCO3)2 and Mg(HCO3)2 salts