Read the following passage and choose the correct answer to each of the questions.

Carbon monoxide poisoning kills and injures many people and animals around the world. The gas has been a problem since people first began burning fuels to cook food or to create heat. It is a problem in all parts of the world that experience cold weather.

Carbon monoxide is called the silent killer because people do not know it is in the air. The gas has no color. It has no taste. It has no smell. It does not cause burning eyes. And it does not cause people to cough. But it is very deadly. It robs the body of its ability to use oxygen.

Carbon monoxide decreases the ability of the blood to carry oxygen to body tissues. It does this by linking with the blood. When the gas links with the blood, the blood is no longer able to carry oxygen to the tissues that need it.

Damage to the body can begin very quickly from large amounts of carbon monoxide. How quickly this happens depends on the length of time a person is breathing the gas and the amount of the gas he or she breathes in.

Carbon monoxide poisoning has warning signs. But people have to be awake to recognize them. Small amounts of the gas will cause a person’s head to hurt. He or she may begin to feel tired. The person may feel sick. The room may appear to be turning around. The person may have trouble thinking clearly. People develop severe head pain as the amount of gas continues to enter their blood. They will begin to feel very tired and sleepy. They may have terrible stomach pains.

Medical experts say carbon monoxide affects people differently. For example, a small child will experience health problems or die much quicker than an adult will. The general health of the person or his or her age can also be important. An older adult with health problems may suffer the effects of carbon monoxide more quickly than a younger person with no health problems. People with heart disease may suffer chest pains. They may begin to have trouble breathing.

It gets (A)more and most difficult (B)to understand what (C)the professor (D)has said.