Final answer:
Combustibles such as wood, paper, and plastics that leave ashes when burned are classified as Class A fires, where the total mass remains constant before and after combustion due to the Law of Conservation of Mass.
Step-by-step explanation:
The fire class that includes combustibles such as wood, paper, plastics, or any other materials that leave ashes is known as Class A fires. These materials are common in everyday environments and are primarily made of carbon-based compounds. When these materials burn, the reaction combines with oxygen and changes to ashes, carbon dioxide, and water vapor. This process is an example of fuel combustion, which is a chemical reaction that releases heat. According to the Law of Conservation of Mass, the total mass of matter after the fire is the same as before, since the matter is neither created nor destroyed, it just changes in form.