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  3. First Planet from the Sun Mercury, the planet nearest the sun, is difficult to observe from the Earth because it rises and s

First Planet from the Sun Mercury, the planet nearest the sun, is difficult to observe from the Earth because it rises and s

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问题                     First Planet from the Sun
    Mercury, the planet nearest the sun, is difficult to observe from the Earth because it rises and sets within two hours of the sun. ______【46】
    Planetary scientists can estimates the age of a planet’s surface by the number of impact craters on it. In general, the older the surface, the more craters it has. ______【47】. Between these regions are areas of gently rolling plains that may have been smoothed by volcanic lava flows or by accumulated deposits of fine material ejected during impacts. These plains are also old enough to have accumulated a large number of impact craters. Elsewhere on the planet are smooth, flat plains that are probably younger and volcanic in origin. ______【48】. Sometime between the formation of the intercrater plains and the formation of the smooth plains, the whole planet may have shrunk as it cooled, causing the crust to buckle and form the long, steep cliffs called scarps.
    The largest impact basin on Mercury has a diameter of about 800 miles (1, 300 kilometers) and is surrounded by mountains that rise to heights of about 1. 2 miles (2 kilometers). ______【49】.
    On the opposite side of the planet is an area of hilly, linear terrain that probably resulted from seismic waves caused by the same impact.
    Like other airless, solid bodies in the solar system, the entire surface of Mercury is covered with a layer of rubble called regolith, which is composed of material, ranging from dust to boulders, that was scattered when impact craters were formed. This debris was in turn broken up and redistributed by subsequent impacts.
    Mercury is very dense and has a magnetic field that is about 1 percent as strong as earth’s. This suggests the existence of a planetary core composed of iron and nickel and constituting about 40 percent of the planet’s volume. ______【50】. Radar images taken of Mercury in 1991 show what are considered to be large ice patches at the planet’s North Pole.
A. Some regions on Mercury are heavily cratered, suggesting that they are very old surfaces that were probably formed about 4 billion years ago
B. The crater was probably created by the impact of a large planetesimal when Mercury was forming
C. Consequently, little was known about the planet until the Mariner 10 spacecraft made several flybys in 1974 and 1975
D. Mercury is much smaller than the Earth
E. The surface gravity is about one-third as strong as Earth’s, and a thin atmosphere surrounds the planet
F. These plains have relatively few impact craters
选项
答案A
解析 前一句提到：“通常，其表面存在的时间越长，其撞击坑就越多。”选项A“水星上的某些地区撞击坑密布，这意味着这些表面存在的时间很久远，大概在40亿年前就形成了”，正是利用这一规律印证的事实。并且“某些地区”与下语句的“这些地区”产生对应关系。所以选项A是正确答案。
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