Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 7 to 11. 

     The common cold is the world's most widespread illness, which is plagues that flesh receives. The most widespread fallacy of all is that colds are caused by cold. They are not. They are caused by viruses passing on from person to person. You catch a cold by coming into contact, directly or indirectly, with someone who already has one. If cold causes colds, it would be reasonable to expect the Eskimos to suffer from them forever. But they do not. And in isolated arctic regions, explorers have reported being free from colds until coming into contact again with infected people from the outside world by way of packages and mail dropped from airplanes. 

     During the First World War soldiers who spent long periods in the trenches cold and wet showed no increased tendency to catch colds. In the Second World War, prisoners at the notorious Auschwitz concentration camp, naked and starving, were astonished to find that they seldom had colds. 

     At the Common Cold Research Unit in England, volunteers took part in experiments in which they gave themselves to the discomforts of being cold and wet for long stretches of time. After taking hot baths, they put on bathing suits, allowed themselves to be sipped with cold water, and then stood about dripping wet in drafty room. Some wore wet socks all day while others exercised in the rain until close to exhaustion. Not one of the volunteers came down with a cold unless a cold virus was actually dropped in his nose. 

     If, then, cold and wet have nothing to do with catching cold, why are they more frequent in the winter? Despite the most painstaking research, no one has yet found the answer. One explanation offered by scientists is that people tend to stay together indoors more in cold weather than at other times, and this makes it easier for cold viruses to be passed on. No one has yet found a cure for the cold. There are drugs and pain suppressors such as aspirin, but all they do is to relieve the symptoms. 

The reading passage mainly discusses _________.