The correct answer is B) prolonged exposure to high concentrations of CO2 gas causes suffocation. Carbon dioxide (CO2) is an effective fire extinguishing agent because it displaces oxygen, thereby depriving the fire of the necessary element to sustain combustion. However, in an enclosed space, the high concentration of CO2 can quickly lead to oxygen deficiency, resulting in suffocation of individuals present. This is a key disadvantage of using CO2 in such environments, as the safety of personnel must be the primary concern. The other answer choices are incorrect. Option A is incorrect because CO2 is actually heavier than air, not lighter. Option C is incorrect as the 'snow' discharged with CO2 is just solidified CO2, not a toxic substance. Option D is incorrect as the rapid dissipation of CO2 vapor is not a disadvantage, but rather a characteristic of the agent.