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长期培训计划一般指时间跨度3～5年及以上的培训计划。()
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According to the prescriptive rule, which of the following sentences would be considered a
It can be inferred from the passage that studies attempting to use tree-ring thickness to
It had been known for many decades that the appearance of sunspots is roughly periodic with an average cycle of eleven years. Moreover, the incidence of solar flares and the flux of solar cosmic rays, ultraviolet radiation and x-radiation all vary directly with the sunspot cycle. But after more than a century of investigation, the relation of these and other phenomena, known collectively as the solar-activity cycle, to terrestrial weather and climate remains unclear. For example the sunspot cycle and the allied magnetic-polarity cycle have been linked to periodicities discerned in records of such variables as rainfall, temperature, and winds. Invariably, however, the relation is weak, and commonly of dubious-statistical significance.
Effects of solar variability over longer terms have also been sought. The absence of recorded sunspot activity in the notes kept by European observers in the late seventeenth and early eighteenth centuries has led some scholars to postulate a brief cessation of sunspot activity at that time (a period called the Maunder minimum). The Maunder minimum has been linked to a span of unusual cold in Europe extending from the sixteenth to the early nineteenth centuries. The reality of the Maunder minimum has yet to be established, however, especially since the records that Chinese naked-eye observers of solar activity made at that time appear to contradict it. Scientists have also sought evidence of long-term solar periodicities by examining indirect climatological data, such as fossil records of the thickness of ancient tree rings. These studies, however, failed to link unequivocally terrestrial climate and the solar-activity cycle, or even to confirm the cycle's past existence.
If consistent and reliable geological or archaeological evidence tracing the solar-activity cycle in the distant past could be found, it might also resolve an important issue in solar physics: how to model solar activity. Currently, there are two models of solar activity. The first supposes that the Sun's internal motions (caused by rotation and convection)interact with its large-scale magnetic field to produce a dynamo, a device in which mechanical energy is converted into the energy of a magnetic field. In short, the Sun's large-scale magnetic field is taken to be self-sustaining, so that the solar-activity cycle it drives would be maintained with little overall change for perhaps billions of years. The alternative explanation supposes that the Sun's large-scale magnetic field is a remnant of the field the Sun acquired when it formed, and is not sustained against decay. In this model, the solar mechanism dependent on the Sun's magnetic field runs down more quickly. Thus, the characteristics of the solar-activity cycle could be expected to change over a long period of time. Modern solar observations span too short a time to reveal whether present cyclical solar activity is a long-lived feature of the Sun, or merely a transient phenomenon.
The author focuses primarily on______.
How the Smallpox War Was Won
The world's last known case of smallpox was reported in Somalia, the Horn of Africa, in October 1977. The victim was a young cook called Ali Maow Maalin. His case becomes a landmark in medical history, for smallpox is the first communicable disease ever to be eradicated.
The remarkable campaign to free the world of smallpox has been led by the World Health Organization. The Horn of Africa, embracing the Ogaden region of Ethiopia and Somalia, was one of the last few smallpox ridden areas of the world when the WHO—sponsored Smallpox Eradication Programme (SEP)got underway there in 1971.
Many of the 25 million inhabitants, mostly farmers and nomads living in a wilderness of desert, bush and mountains, already had smallpox. The problem of tracing the disease in such formidable country was exacerbated further by the continuous warfare in the area.
The program concentrated on an imaginative policy of "search and containment." Vaccination was used to reduce the widespread incidence of the disease, but the success of the campaign depended on the work of volunteers. These were men, paid by the day, who walked hundreds of miles in search of "rumours”—information about possible smallpox cases.
Often these rumours turned out to be cases of measles, chicken pox or syphilis but nothing could be left to chance. As the campaign progressed the disease was gradually brought under control. By September 1976 the SEP made its first report that no new cases had been reported. But that first optimism was short-lived. A three-year-old girl called Amina Salat, from a dusty village in the Ogaden in the south-cast of Ethiopia, had given smallpox to a young nomad visitor. Leaving the village the nomad had walked across the border into Somalia. Therc he infected 3,000 people, and among them had been the cook, Ali. It was a further 14 months before the elusive "target zero"—no further cases--was reached.
Even now, the search continues in "high risk" areas and in parts of the country un- checked for some time. The flow of romours has now diminished to a trickle--but each must still be checked by a qualified person.
Victory is in sight, but two years must pass since the last case before an International Commission can declare that the world is entirely free from smallpox.
Ali Maow Maalin's case is significant because he was the______.
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