单选题 A tree's age can be determined by counting the annual growth rings in its trunk. Each ring represents one year, and the ring's thickness reveals the relative amount of rainfall that year. Archaeologists successfully used annual rings to determine the relative ages of ancient tombs at Pazyryk. Each tomb was constructed from freshly cut logs, and the tombs' builders were constrained by tradition to use only logs from trees growing in the sacred Pazyryk Valley.
Which one of the following, if true, contributes most to an explanation of the archaeologists' success in using annual rings to establish the relative ages of the tombs at the Pazyryk site?

A、 The Pazyryk tombs were all robbed during ancient times, but breakage of the tombs' seals allowed the seepage of water, which soon froze permanently, thereby preserving the tombs' remaining artifacts.
B、 The Pazyryk Valley, surrounded by extremely high mountains, has a distinctive yearly pattern of rainfall, and so trees growing in the Pazyryk Valley have annual rings that are quite distinct from trees growing in nearby valleys.
C、 Each log in the Pazyryk tombs has among its rings a distinctive sequence of twelve annual rings representing six drought years followed by three rainy years and three more drought years.
D、 The archaeologists determined that the youngest tree used in any of the tombs was 90 years old and that the oldest tree was 450 years old.
E、 (E) All of the Pazyryk tombs contained cultural artifacts that can be dated to roughly 2300 years ago.

【正确答案】 C

【答案解析】[解析] 本题的结论是考古学家们利用年轮已成功地确定了Pazyryk地区的古墓的相对年龄。要使用年轮来确定古墓的相对年龄，首先就要考虑具有这些年轮的木料之间有无相关性，即它们之间有无一致的地方。在5个选项中，只有(C)指出了这种相关性，即Pazyryk地区的墓中的每一块木料的年轮中都有一个有特色的12年龄序列，这也就是说Pazyryk地区的古墓修建时用的木料在生长过程中都经历了6年的干旱，接着是3年的多雨和3年的干旱，在用年轮来确定墓的相对年龄时，只要看一下这12个有特色序列的年轮在整个木料的年轮中的位置即可。若古墓用的木料中，这12个年轮位于树干的靠中间的位置，则此墓建得较晚；若这12个年轮位于树干的靠周边的位置，则此墓建得较早。根据这12个年轮在古墓的木料中出现的位置的差异，就可确定出古墓之间的相对年龄。本题的(A)、(B)、(D)和(E)选项都是与本题的结论无关的选项。