考生舉行GRE瀏覽測驗的備考，恰當的演習是必弗成少的，本日給考生分享的是GRE瀏覽真題200篇LEVEL2:It has been known for many decades...，願望須要的考生可如下載下來然後模仿演習。
52. It has been known for many decades
It has 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.
17. The author focuses primarily on
(A) presenting two competing scientific theories concerning solar activity and evaluating geological evidence often cited to support them
(B) giving a brief overview of some recent scientific developments in solar physics and assessing their impact on future climatological research
(C) discussing the difficulties involved in linking terrestrial phenomena with solar activity and indicating how resolving that issue could have an impact on our understanding of solar physics
(D) pointing out the futility of a certain line of scientific inquiry into the terrestrial effects of solar activity and recommending its abandonment in favor of purely physics-oriented research
(E) outlining the specific reasons why a problem in solar physics has not yet been solved and faulting the overly theoretical approach of modern physicists
18. Which of the following statements about the two models of solar activity, as they are described in lines 37-55, is accurate?
(A) In both models cyclical solar activity is regarded as a long-lived feature of the Sun, persisting with little change over billions of years.
(B) In both models the solar-activity cycle is hypothesized as being dependent on the large-scale solar magnetic field.
(C) In one model the Sun’s magnetic field is thought to play a role in causing solar activity, whereas in the other model it is not.
(D) In one model solar activity is presumed to be unrelated to terrestrial phenomena, whereas in the other model solar activity is thought to have observable effects on the Earth.
(E) In one model cycles of solar activity with periodicities longer than a few decades are considered to be impossible, whereas in the other model such cycles are predicted.
19. According to the passage, late seventeenth and early eighteenth-century Chinese records are important for which of the following reasons?
(A) They suggest that the data on which the Maunder minimum was predicated were incorrect.
(B) They suggest that the Maunder minimum cannot be related to climate.
(C) They suggest that the Maunder minimum might be valid only for Europe.
(D) They establish the existence of a span of unusually cold weather worldwide at the time of the Maunder minimum.
(E) They establish that solar activity at the time of the Maunder minimum did not significantly vary from its present pattern.
20. The author implies which of the following about currently available geological and archaeological evidence concerning the solar-activity cycle?
(A) It best supports the model of solar activity described in lines 37-45.
(B) It best supports the model of solar activity described in lines 45-52.
(C) It is insufficient to confirm either model of solar activity described in the third paragraph.
(D) It contradicts both models of solar activity as they are presented in the third paragraph.
(E) It disproves the theory that terrestrial weather and solar activity are linked in some way.
21. It can be inferred from the passage that the argument in favor of the model described in lines 37-45 would be strengthened if which of the following were found to be true?
(A) Episodes of intense volcanic eruptions in the distant past occurred in cycles having very long periodicities.
(B) At the present time the global level of thunderstorm activity increases and decreases in cycles with periodicities of approximately 11 years.
(C) In the distant past cyclical climatic changes had periodicities of longer than 200 years.
(D) In the last century the length of the sunspot cycle has been known to vary by as much as 2 years from its average periodicity of 11 years.
(E) Hundreds of millions of years ago, solar-activity cycles displayed the same periodicities as do present-day solar-activity cycles.
22. It can be inferred from the passage that Chinese observations of the Sun during the late seventeenth and early eighteenth centuries
(A) are ambiguous because most sunspots cannot be seen with the naked eye
(B) probably were made under the same weather conditions as those made in Europe
(C) are more reliable than European observations made during this period
(D) record some sunspot activity during this period
(E) have been employed by scientists seeking to argue that a change in solar activity occurred during this period
23. It can be inferred from the passage that studies attempting to use tree-ring thickness to locate possible links between solar periodicity and terrestrial climate are based on which of the following assumptions?
(A) The solar-activity cycle existed in its present form during the time period in which the tree rings grew.
(B) The biological mechanisms causing tree growth are unaffected by short-term weather patterns.
(C) Average tree-ring thickness varies from species to species.
(D) Tree-ring thicknesses reflect changes in terrestrial climate.
(E) Both terrestrial climate and the solar-activity cycle randomly affect tree-ring thickness.
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