GED Science Practice

The correct answer is carbon dioxide, which is a heavy, odorless, colorless gas produced during the process of respiration in living organisms, including humans. During respiration, cells convert glucose and oxygen into energy, releasing carbon dioxide as a byproduct. This gas is also generated during the decomposition of organic substances, as microorganisms break down the material and release carbon dioxide in the process.

Carbon dioxide plays a crucial role in the Earth's atmosphere, serving as a greenhouse gas and being critical for photosynthesis in plants. Unlike carbon monoxide, which is a harmful gas produced by incomplete combustion of carbon-containing substances, carbon dioxide is a natural and essential component of the respiratory cycle.

Other options, like carcinoma and catalyst, are unrelated to the decomposition process or respiration. Carcinoma refers to a type of cancer, which is not a gas, while a catalyst is a substance that speeds up a chemical reaction without undergoing permanent change itself, not a gas produced during respiration or decay.