Explanation:

Coal and petroleum are fossil fuels formed from the decomposition of organic matter over millions of years. Their primary chemical composition consists of hydrocarbons, which are compounds made up of hydrogen and carbon atoms. Therefore, they are enormous reservoirs of carbon. When these fuels are burned, they release large amounts of carbon dioxide ($CO_2$) into the atmosphere, contributing to the carbon cycle and climate change. Options A (ozone), B (calcium), and D (zinc) are not primary components or major reservoirs within coal and petroleum.