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46 . How the diseases are expressed depends on how the particular environmental agent enters the body, how it is metabolized(引起代谢)，and by what route it is excreted. The skin, lungs, liver, kidneys, and 47 . Of particular concern is the capacity of many environmental agents to cause various cancers, birth defects or spontaneous affection (through fetal exposure), and mutations(突变)in germ cells, 48 . 
Environmental illnesses can be mild or severe and range from transient to chronic, depending on the doses of toxin(毒素)received. Some diseases occur abruptly at a toxic exposure, whereas the time of onset of other diseases varies after exposure. 49 . Those illnesses that occur directly after a distinct toxic exposure are usually easily identified as being environmentally or occupationally caused. If the exposure is not clear-cut or illness is delayed, however, the cause is difficult to identify, as clinical features alone are usually nonspecific. 50 . In such instances, epidemiological studies of exposed populations can help relate exposures to the illnesses they cause. 
A Environmentally induced cancers, for example, commonly involve latency (潜伏期)periods of 15 to 30 years or more
B In addition, many different causes, environmental or otherwise, may produce identical illnesses
C However, the reverse is true
D nervous systems are commonly affected by different agents in different settings
E the lastnamed raising possibilities of environmentally caused genetic diseases in later generations
F Environmental diseases can affect any organ system of the body
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Taking a Nap During the Day
Medical experts say most Americans do not get 51 sleep. They say more Americans need to rest for a short period in the middle of the day. They are advising people to sleep lightly before 52 with other activities.
One study earlier this year found that /www.yingyukaoshi.com/ persons who sleep for a few minutes during the day were less 53 to die of heart disease. The study followed more than 2,300 Greek adults 54 about six years. Adults who rested for half an hour 55 three times a week had a 37 present lower risk of dying from heart disease than those who did not nap.
Study organizers said the strongest evidence was in working men. They said naps might improve 56 by mitigating tension caused by work.
Some European and Latin American businesses have supported the 57 of napping for many years. They urge people to 58 work, go home and have a nap before returning. In the United States, some companies let workers rest briefly in their offices. They believe this reduces 59 and accidents, and 60 increases the amount of work a person can do.
Sleep experts say it is likely that people make more mistakes at work than at other times. They say people should not carry out important duties when they feel 61 And they say the best thing to do is to take a nap. About twenty minutes of rest is 62 you need. Experts say this provides extra energy and can increase your effectiveness 63 the end of the day. But experts warn that a nap 64 last no more than twenty to thirty minutes. A longer nap will put the body into deep sleep and waking up will be 65 .
51. A sweet B sound C bad D enough
52. A checking B sharing C continuing D meeting
53. A lovely B likely C fondly D finely
54. A for B at C in D with
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