【正确答案】 A

【答案解析】[听力原文] 16-20
Now, let's move on to fire safety. Before I talk to you about safety precautions and procedures, I'd like to mention some of the effects of smoke and heat on humans. There are four ingredients of fire, namely: oxygen, fuel, heat and chain reactions. Almost all materials burn and most household goods burn very easily. The air we breathe contains about 21% oxygen. As fire burns it consumes oxygen, thereby reducing the oxygen content of the air. When that is reduced to 15%, oxygen deficiencies in body tissue cause an impairment of muscle control and dexterity. At between 10 and 14 percent judgment and reasoning are affected. Oxygen reduction to between 6 and 10 percent results in unconsciousness and breathing stops. Sounds scary, doesn't it? But that's not all you have to worry about.
Many materials in the home give off toxic gases as they burn. The main toxic gases are carbon monoxide, carbon dioxide, hydrogen sulphide and nitrogen oxide. Carbon monoxide is an invisible and odourless gas produced in all fires. It takes up the place of oxygen in the blood thereby reducing the supply of oxygen to the brain. The effect of carbon dioxide is to increase the heart rate and stimulate the rate of breathing, causing an increase in the intake of other toxic gases which contributes to, amongst other symptoms, serious oxygen deprivation.
Hydrogen sulphide, on the other hand, affects the nervous system, causing dizziness and pain in the respiratory system. It does occur naturally in volcanic gases and hot springs and it also results from the bacterial breakdown of organic matter. You're probably familiar with it—you know, it has that characteristic odour of rotten eggs. But, make no mistake, in large concentrations, it's deadly. Lastly, nitrogen oxide is another extremely poisonous gas at high levels of concentration which deadens feeling in the throat and lungs, causes swelling in the throat and a buildup of fluids in the lungs.