A、People with low incomes. B、Researchers exposed to harmful substances. C、People having family problems. D、People under the age

There are many explanation of why people catch colds. Some say colds are caused by the environment. Others say colds can result from smoking too much, lack of exercise, not enough rest, poor diet, or working too hard. But new studies suggest that people who feel extreme tension for a long period of time also may be at a higher risk of developing a cold. The researchers note that stress is not the cause of all colds. But studies show that people who suffer from the stress caused by serious personal problems are more likely to get sick than those who do not. Researchers have spent years trying to find out why some people have colds more often than others. An early study directed in 1991 showed that emotional stress can lead to colds. In current studies, researchers at the University of Pittsburgh studied almost 300 people, aged 18 to 55. They were given physical, social and mental and emotional examinations. Then live cold viruses were placed in their noses. During the next five days, they were examined to find out who became infected by the virus and then developed signs of a cold. Such signs include coughing, sneezing and a blocked nose. Stress lowers resistance, but just how it does is unknown. So in this study, the researchers look for the kinds of stress involved and how they might have affected resistance to colds. The study found that severe stress lasting a month or more increased the risk of a cold. Two causes of stress ― losing a job or having difficulties with family members or friends ― increase the risk the most.
Questions 33 to 35 are based on the passage you have just heard.
33. What’s the passage mainly about?
34. What can we know from the study directed in 1991?
35. What kind of people are liable to develop a cold?