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.