【正文】 s core. ? Lighter elements from Earth39。 thus, even continental crust reaches only about 1/200th of the way to Earth39。 but once Earth was fully formed, it was large enough for its gravitational field to hold most of the remaining gases, and these formed Earth39。 thus, even continental crust reaches only about 1/200th of the way to Earth39。s spin and tilt are unique among other plaary bodies in the solar system 6. The word constant in the passage is closest in meaning to ? Considerable ? Unpredictable ? Continual ? Violent Paragraph 2: As it began to reach full size, Earth heated up, partly because of collisions with other plaesimals and partly because of increasing internal pressures as it grew in size. In addition, the early Earth contained abundant radioactive materials, also a source of heat. As Earth heated up, its interior melted. Within the molten interior, under the influence of gravity, different elements were sorted out by density. By about 40 million years after the formation of the solar system, most of the heavier metallic elements in the early Earth, such as iron and nickel, had sunk through the hot sludge to the center giving Earth a core dominated by iron. This metallic core gives Earth its characteristic magic field, which has played an extremely important role in the history of our pla. 7. Paragraph 2 answers which of the following questions about early Earth ? What caused materials on Earth to bee radioactive ? What percentage of Earth39。s interior. These lighter materials, such as the rocks we call granites, formed a layer of continental crust about 35 kilometers thick. Relative to Earth as a whole, this is as thin as an eggshell. Seafloor crust is even thinner, at about 7 kilometers。 they were found on a mountainside in British Columbia in a deposit known as the Burgess Shale. The Burgess Shale fossils are extraordinarily important because among them are remains of softbodied creatures, many of them lacking shells and other hard parts that fossilize easily. Consequently, their preservation is little short of miraculous (as are the delicate methods used to reconstruct threedimensional structure from these flattened fossils), and they are one of the few known repositories of early softbodied animals. 7. The phrase little short of miraculous is closest in meaning ? To very highly valued ? Amazing because almost impossible ? Causing controversy ? Almost but not quite plete 8. According to paragraph 4, all of the following are true of the Burgess Shale EXCEPT: ? Its fossils were in a flattened condition when discovered. ? Its fossils provide direct evidence about the origin of eyes. ? It contains fossils of both Precambrian and Cambrian animals. ? It contains fossilized remains of softbodied anisms. Paragraph 5: Not all of the Burgess animals had eyes. However, some did. (Gross features location, size, and hemispheric shape are responsible for the designation of some structures as eyes). The reconstructed eyes of these Burgess animals look superficially like eyes of some living crustaceans, particularly those of shrimp and crabs whose eyes are mounted on stalks that improve the range of vision by raising the eyes above the surface of the head. The eyes of some Burgess anisms sat on stalks。 those of others were on or a part of the body surface. One animal, Opabinia, had five eyes: two lateral pairs and a single medial eye。s mantle, a region almost 3,000 kilometers thick between the core and the crust. With the help of bombardment by ets, whose many impacts scarred and heated Earth39。 but once Earth was fully formed, it was large enough for its gravitational field to hold most of the remaining gases, and these formed Earth39。s surface, the lightest silicates rose to Earth39。s size ? caused the temperatures of Earth39。s surface, the lightest silicates rose to Earth39。s interior rose and formed the mantle, a denser layer of silicates around the core, and the crust, a thinner layer of silicates at Earth39。 however, there were not yet efficient means by which to produce mechanical energy or to power machinery. This was to occur with progress in the development of the steam engine. In the late 1700s James Watt designed an efficient and mercially viable steam engine that was soon applied to a variety of industrial uses as it became cheaper to use. The engine helped solve the problem of draining coal mines of groundwater and increased the production of coal needed to powe。s lack of an atmosphere explains why it was bombarded with much more frequency and violence than other plaesimals. ? Continued bombardments and internal pressures made the growing Earth hotter, causing its interior to melt and the heavier elements to sink and form Earth39。s surface. ? It lacked water vapor. Paragraph 3: As heavy materials headed for the center of Earth, lighter silicates (such as the mineral quartz) drifted upward. The denser silicates formed Earth39。s continental crust ? has changed significantly in position over time ? was as thick as Earth39。s core is mostly iron because, pared to most other elements on early Earth, iron ? was denser ? melted more easily ? was more radioactive ? was more plentiful Paragraph 3: As heavy materials headed for the center of Earth, lighter silicates (such as the mineral quartz) drifted upward. The denser silicates formed Earth39。s surface to the present day. The lightest materials of all, including gases such as hydrogen and helium, bubbled through Earth39。 they seem much too large and (potentially) well developed to be brand new inventions. The best we can do is put the origin of eyes somewhere between the beginning of the Cambrian explo sion, about 600 million years ago, and the death of the Burgess animals, some 530 million years ago