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lo and Europa, the inner two of Jupiter's four largest moons, are about the size of Earth’s moon and are composed mostly or entirely of rock and metal. Ganymede and Callisto are larger and roughly half ice. Thus, these four moons are somewhat analogous to the planets of the solar system, in which the rock- and metal-rich inner planets are distinct from the much larger gas- and ice-rich outer planets. Jupiter's moons are, however, more "systematic" :many of their properties vary continuously with distance from Jupiter. For example, lo is ice-free, Europa has a surface shell of ice, and while Ganymede and Callisto are both ice-rich, outermost Callisto has more.This compositional gradient has geological parallels, lo is extremely geologically active, Europa seems to be active on a more modest scale, and Ganymede has undergone bouts of activity in its geological past. Only Callisto reveals no geological activity. In similar fashion, Callisto's surface is very heavily cratered from the impact of comets and asteroids; Ganymede, like Earth’s moon, is heavily cratered in parts; Europa is very lightly cratered; and no craters have been detected on lo, even though Jupiter's gravity attracts comets and asteroids passing near it, substantially increasing the bombardment rate of the inner moons compared to that of the outer ones. But because of lo's high degree of geological activity, its surface undergoes more-or-less continuous volcanic resurfacing.
The author's reference to Jupiter's gravity in highlight serves primarily to
A indicate why the absence of craters on lo's surface is surprising
B explain the presence of craters on the surface of Jupiter's four largest moons
C provide an explanation for the lack of geological activity on Callisto
D contrast Jupiter’s characteristics with the characteristics of its four largest moons
E illustrate the similarity between Jupiter's four largest moons and the planets of the solar system
【申友解析】
细节题，定位依据25行。解题思路，此处解题的关键是even though所表示的让步关系。作者借此提出了奇怪的现象：Io离木星距离近，按照常理，由于近木星区域彗星和小行星活动频繁，Io表面的坑洞应该“星罗棋布”才是，但实际上其表面却没有探测到坑洞。
A正确。符合第二段此处的让步的目的，即提出一个奇怪的现象；
B错误。并非要explain，而是要提出一个违反常理的现象。
C错误。同B
D错误。此选项容易混淆，但文中这句话最主要的目的还是想说明Io表面没有坑洞的奇怪之处，而并非将木星的特点与四大卫星作比较。
E错误。与太阳系的行星的特点无关，这是第一段的相关内容
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