题目解析：
本题为功能目的题。功能目的题中，作者提到的具体信息一般是为了证明或反驳某个观点。我们首先在文中找到句子的出处。“In fact Moscow has been described as the first stopping and setting point in the northeast. But it also profited。from moments in other directions……”这句话中，“in fact”说明阴影句是对上文内容的补充说明，而“but”带有转折关系，说明下文会展开新的层次，所以答案一定不在后文中，而是在“in fact”之前。于是我们段首开始看，“First, attention may be given to the doctrine of geographic causation……and includes several lines of argument.”这句话是本段的主旨句，强调地理因素对莫斯科崛起的重要性。而“Moscow lay as a crossing of three roads……as the first stopping and setting point in the northeast.”含阴影句在内的这一段内容，都是为了说明主旨而阐述的细节性信息。那么我们可以很明显地抓住阴影句和主旨句之间的关系，即阴影句是为了证明和强调主旨。选项A说阴影句为解释为什么莫斯科周围的地区衰落，而文章中只提到“The most important was the way from the historically crucial city of Kiev and the declining south to the growing northeast.”说明莫斯科连接了衰落的西部和发展中的东北部，同样也是在强调莫斯科地理位置的优越性，与阴影句不存在因果关系，故排除。
B选项和D选项定位到文章中看，都是“but……”之后的内容，与阴影句几乎没有关联，可以直接排除。
C选项说阴影句是为了强调莫斯科优越的地理位置对其发展的重要性，很明显抓住了与主旨之间的关系，故为正确答案。
First, attention may be given to the doctrine of geographic causation. It stresses the decisive importance of the location on Moscow for the later expansion of the Muscovite state (the medieval state centered in Moscow) and includes several lines of argument. Moscow lay as a crossing of three roads. The most important was the way from the historically crucial city of Kiev and the declining south to the growing northeast. In fact Moscow has been described as the first stopping and setting point in the northeast . But it also profited from moments in other directions, including the reverse. Thus it seems immigrants came to Moscow after the Mongol devastation of the lands further to the northeast. Moscow was also situated on a bend of the Moscow River that flows from the northwest to the southeast into the Oka, the largest western tributary of the Volga River. To speak more broadly of water communications which span and unite European Russia, Moscow has the rare fortune of being located near the headwaters of four major rivers: the Oka, the Volga, the Don, and the Dnieper. This offered marvelous opportunities for expansion across the flowing plain, especially as there were no mountains or other natural obstacles to hem in the young principality.
