要构成一个完整的计算机系统，需要什么？

Visitors to this country are normally admitted for six months, but foreign students can usually stay for one year. They must (54) an educational institution, and they are required to study for a (55) of fifteen hours a week on a daytime course. Prospective students have to show that they can afford their studies, and that they have sufficient (56) resources to support themselves (57) in this country.

In order to work here the foreigner needs a work permit, (58) must be applied for by his prospective employer. The problem here is that the Department of Employment has the right to grant or (59) these permits, and there is little that can be done (60) it; it would be extremely unwise for a foreign visitor to work (61) a permit, since anyone doing so is liable to immediate deportation (驱逐出境). There are some people from the European countries, who are often given (62) residence permits of up to five years. Some other people, such as doctors, foreign journalists, authors and others can work without permits, and foreign students are normally allowed to (63) part-time jobs while they are studying here.

55().

A.minimum 

B.maximum 

C.minority 

D.majority

主动脉瓣中度反流时，反流束宽度／左心室流出道宽度为（）

A.47%～64%

B.25%～46%

C.20%～25%

D.15%～20%

E.10%～15%

巩膜三大薄弱处位于（）。

A.角巩膜缘、赤道部、巩膜筛板

B.角巩膜缘、肌肉附着点、赤道后

C.角巩膜缘、肌肉附着点、巩膜筛板

D.巩膜筛板、赤道部、赤道后

E.肌肉附着点、赤道部、赤道后

硬膜外追加局麻药的时机可参照（）

A.距上次给药时间

B.骨骼肌松转紧

C.阻滞平面下移

D.手术部位由无痛转为有痛

E.内脏牵拉反应由轻变重

Increasingly, historians are blaming diseases imported from the Old World for the great disparity between the native population of America in 1492--new estimates of which jump as high as 100 million, or approximately one-sixth of the human race at that time--and the few million full-blooded Native Americans alive at the end of the nineteenth century. There is no doubt that chronic disease was an important factor in the sharp decline, and it is highly probable that the greatest killer was epidemic disease, especially as manifested in virgin-soil epidemics.

Virgin-soil epidemics are those in which the populations at risk have had no previous contact with the diseases that strike them and are therefore immunologically almost defenseless. That virgin-soil epidemics were important in American history is ply indicated by evidence that a number of dangerous maladies--smallpox, measles, malaria, yellow fever, and undoubtedly several more--were unknown in the pre-Columbian New World. The effects of their sudden introduction are demonstrated in the early chronicles of America, which contain reports of horrible epidemics and steep population declines, confirmed in many cases by quantitative analyzes of Spanish tribute records and other sources. The evidence provided by the documents of British and French colonies is not as definitive because the conquerors of those areas did not establish permanent settlements and began to keep continuous records until the seventeenth century, by which time the worst epidemics had probably already taken place. Furthermore, the British tended to drive the native populations away, rather than to enslave them as the Spaniards did; so that the epidemics of British America occurred beyond the range of colonists’ direct observation.

Even so, the surviving records of North America do contain references to deadly epidemics among the native population. In 1616--1619 an epidemic, possibly of pneumonic plague, swept coastal New England, killing as many as nine out of ten. During the 1630’s smallpox, the disease most fatal to the Native American people, eliminated half the population of the Huron and Iroquois confederations. In the 1820’s fever ruined the people of the Columbia River area, killing eight out of ten of them.

Unfortunately, the documentation of these and other epidemics is slight and frequently unreliable, and it is necessary to supplement what little we de know with evidence from recent epidemics among Native Americans. For example, in 1952 an outbreak of measles among the Native American inhabitants of Ungava Bay, Quebec, affected 99 percent of the population and killed 7 percent, even though some had the benefit of modern medicine. Cases such as this demonstrate that even diseases that are not normally fatal can have destroying consequences when they strike an immunologically defenseless community.

Notes: disparity 差距。 virgin-soil处女地。 malady 疾病 chronicle 编年史。 tribute 贡品。 pneumonic plague肺鼠疫。confederation 同盟。 smallpox 天花。measles 麻疹。

According to the text, virgin-soil epidemics can be distinguished from other catastrophic outbreaks of disease in that virgin-soil epidemics（）

A. recur more frequently than chronic diseases

B. involve populations with no prior exposure to a disease

C. usually involve a number of interacting diseases

D. are less responsive to medical treatment than are other diseases