Exposure to environmental tobacco smoke (ETS)-also called “second-hand smoke” and “passive smoking” contains over 4 000 chemicals, including over 50 known carcinogens (致癌物) and many poisonous things. No safe level of ETS exposure has been proved. Nor is there any expectation that further research will identify such a level.
Several recent reviews have confirmed the serious risks to health and life associated with passive smoking. Long time exposure to second-hand smoke has been proved as a cause of many of the same diseases caused by active smoking, including lung cancer, cardiovascular (心脏血管的) disease, and childhood disease.
Living with a smoker has been shown to increase the risk of coronary heart disease (冠心病) among non-smokers by 25-30% (a recent study indicates that this figure may actually be higher). 
There is also growing evidence that passive smoking is causally linked to stroke in non-smokers, although further research is needed to estimate the risk.
Second-hand smoke is especially dangerous for young children and infants, being associated with sudden infant death, pneumonia, bronchitis (支气管炎), asthma (哮喘) and respiratory symptoms as well as middle ear disease. ETS exposure in pregnant women can cause lower birth weight, foetal (胎儿的) death and preterm delivery.
Most of the adverse health outcomes brought about by ETS show that the risk increases steadily with increasing exposure. The level of individual risk is lower compared to active smoking. Nevertheless, the fact that large numbers of people are exposed results in a substantial burden of disease.
Action on smoke-free environments would not only protect people from the harm of ETS exposure but also contribute to the reduction of tobacco consumption in the whole population. The health effects of reduced passive and active smoking would include reduced illness and death from major disease types-in particular lung cancer, coronary heart disease, and stroke-and increased life expectancy.
51. Which of the following statements is true? _______
A. Although ETS contains many harmful things, it is not unsafe.
B. The fact that second-hand smoke is safe has been recently found.
C. It is expected that further research will be done to test the level of ETS.
D. By far the safe level of ETS has not been found and will not in the future.
52. Cardiovascular disease can be caused by _______.
A. long time passive smoking B. accidental active smoking
B. occasional passive smoking D. short term active smoking
53. According to the article, the statements about the risk of coronary heart disease are wrong EXCEPT that________
A. if you smoke, the risk of this disease will increase by 25-30%.
B. the non-smokers exposed to smoke will increase the risk of this disease by 25-30%.
C. for the smokers, the risk of getting this disease is higher than that of the non-smokers.
D. the non-smokers exposed to smoke will be more easily to get this disease than the smokers.
54. What does the underlined word “delivery” mean? ________.
A. Transferring to another B. Giving birth C. Throwing away D. Giving out
55. What’s the author’s main purpose in writing this passage? ______
A. To introduce what is ETS.
B. To tell the readers how to avoid ETS.
C. To call for the action against ETS.
D. To warn the readers against smoking.