Travel is exciting and summer is a busy period for holiday travel. Many people will travel great distances in airplanes, cars or other vehicles. Experts say people should know about other health problems that can strike when traveling by air. One of these is a condition called hypoxia  . It results from a lack of oxygen to the brain. Experts say the body begins losing oxygen minutes after an airplane leaves the ground. The air pressure in a plane during flight is lower than at sea level. This makes it more difficult for the body to effectively use the same amount of oxygen as it would on the ground. Fewer oxygen molecules   cross the tissues   in the lungs and reach the bloodstream. The result is a five to twenty percent drop in the amount of oxygen in the blood. This reduces the amount of oxygen that reaches the organs of the body. One effect of this lack of oxygen to the brain is a headache. When this happens, the heart attempts to _ by beating harder and faster. This can make the traveler feel tired. These signs of hypoxia are not dangerous in a healthy person. But a drop in oxygen level can cause a health emergency in people with heart or lung problems. They might lose consciousness or even suffer a heart attack. Experts say that smoking cigarettes and drinking alcoholic liquids also reduce the body's ability to use oxygen. So they suggest that people not drink alcohol or smoke cigarettes either before or during a flight. They also say persons with heart or lung problems should seek advice from their doctors before flying. The author writes the passage mainly to   _  .
warn people in airplanes of possible dangers offer some advice on avoiding health problems explain the causes and effects of hypoxia advise people not to smoke or drink alcohol
Question: Travel is exciting and summer is a busy period for holiday travel. Many people will travel great distances in airplanes, cars or other vehicles. Experts say people should know about other health problems that can strike when traveling by air. One of these is a condition called hypoxia  . It results from a lack of oxygen to the brain. Experts say the body begins losing oxygen minutes after an airplane leaves the ground. The air pressure in a plane during flight is lower than at sea level. This makes it more difficult for the body to effectively use the same amount of oxygen as it would on the ground. Fewer oxygen molecules   cross the tissues   in the lungs and reach the bloodstream. The result is a five to twenty percent drop in the amount of oxygen in the blood. This reduces the amount of oxygen that reaches the organs of the body. One effect of this lack of oxygen to the brain is a headache. When this happens, the heart attempts to _ by beating harder and faster. This can make the traveler feel tired. These signs of hypoxia are not dangerous in a healthy person. But a drop in oxygen level can cause a health emergency in people with heart or lung problems. They might lose consciousness or even suffer a heart attack. Experts say that smoking cigarettes and drinking alcoholic liquids also reduce the body's ability to use oxygen. So they suggest that people not drink alcohol or smoke cigarettes either before or during a flight. They also say persons with heart or lung problems should seek advice from their doctors before flying. The author writes the passage mainly to   _  .
A. warn people in airplanes of possible dangers
B. offer some advice on avoiding health problems
C. explain the causes and effects of hypoxia
D. advise people not to smoke or drink alcohol
Answer: