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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______.
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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______.
When these early migrants arrived in North America, they found the woods and plains dominated by three types of American mammoths. These elephants were distinguished from today's elephants mainly by their thick, shaggy coats and their huge, upward-curving tusks. They had arrived on the continent hundreds of thousands of years before their human followers. The wooly mammoth in the North, the Columbian mammoth in middle North America, and the imperial mammoth of the South, together with their distant cousins the mastodons, dominated the land. Here, as in the Old World, there is evidence that humans hunted these elephants, as shown by the numerous spear points found with mammoth remains.
Then, at the end of the Ice Age, when the last glaciers had retreated, there was a relatively sudden and widespread extinction of elephants. In the New World, both mammoths and mastodons disappeared. In the Old World, only Indian and African elephants survived.
Why did the huge, seemingly successful mammoths disappear？ Were humans connected with their extinction.* Perhaps, but at that time, although they were cunning hunters, humans were still widely scattered and not very numerous. It is difficult to see how they could have prevailed over the mammoth to such an extent.
Which of the following conclusions about mammoths does the passage support？
Why was he wanted in Argentine？
Identifying leadership traits, or the physical and psychological characteristics of leaders, was the first formal approach, and had a lot of intuitive appeal. It owed its origins to the turn of the century(about 1904)when trait studies began. At this time most American leaders came from certain wealthy families, the vast majority were white males, and there were some social norms about what leaders looked like (tall, square jaw, well groomed etc. ). The original assumption that "leaders are born not made" has been discredited, because there were too many exceptions to the traits to give them any credibility. Beginning after World War II, in sharp contrast to the trait approach, the behavioral approach looked at what a leader does, what behaviors leaders use that set them apart from others. This approach assumed that leadership could be learned. Virtually all of the studies focused on classifying behaviors according to whether they fell into a process or "people approach" (satisfying individual needs) , or a "task approach" (getting the job done). The basis for this classification was in the discovery in social psychology that every group needs someone to fill both these roles in the group for it to be effective. The earliest of these studies began in Ohio State University and the University of Michigan in the late 1940s. Many of the early trait and behavioral writers tried to make their ideas applicable to all leadership situations. The earliest situational approach to leadership was developed in 1958. This approach strived to identify characteristics of the situation that allowed one leader to be effective where another was not. The trend later developed toward the third approach, understanding the unique characteristics of a situation and what kind of leadership style. best matches with these.
Which of the following questions does the author answer in the first paragraph？
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