2道CXD解释题求助-答案也看不懂, 请大牛帮忙!

P315-42: 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 thinkness reveals the relative amount of rainfall that year. Archaeologists successfullly used annual rings to determine the relative ages of ancient tombs at Pazyryk. Each tomb was contructed from freshly cut logs, and the tombs' builders were constrained by tradition to use only logs from trees grown in the sacred Pazyryk Valleys.

Which one of the following, if true, contributes most to the explanation of the archaeologists' success in using annual rings to establish the relative ages of the tombs?

A: The Pazyryk tombs were all robbed duing 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 the extremely high mountains, has a distinctive yearly pattern of rainfall, and so trees growing in the valley have annual rings that are quite distinct from trees growing in nearby valley.

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. All of the Pazyryk tombs contained cultural artifacts that can be dated to roughly 2300 years ago.

答案是D, 我不太明白, 树的年轮不仅仅能确认那棵树被砍时候的岁数吗, 比如说树有120岁, 但如何确认古墓的岁数呢?

P319-49: A group of scientists studying calcium metabolism in lab rats discovered that removing the rats' parathyroid glands resulted in the rats' having substantially lower than normal levels of calcium in their blood. The discovery led the scientists to hypothesize that the function of the parathyroid gland is to regulate the level of calcium in the blood by raising that level when it falls below the normal range. In a further experiment, the scientists  removed not only the parathyroid gland but also the adrenal gland from rats. They made the surprising discovery that the level of calcium in the rats' blood decreased much less sharply than when the parathyroid gland alone was removed.

Which one of the following, if true, explains the surprising discovery in a way most consistent with the scientists' hypothesis?

A: The adrenal gland acts to lower the level of calcium in the blood.

B: The Adrenal gland and the parathyroid gland play the same role in the regulating calcium blood levels.

C. The absence of a parathyroid gland causes the adrenal gland to increase the level of calcium in the blood.

D. If the adrenal gland, and no other gland, of a rat were removed, the rat's calcium level would remain stable.

E. The only function of the parathyroid gland is to regulate the level of calcium in the blood.

答案是A, 不明白calcium的下降已经减慢了,为什么还能推出adrenal gland 可以降低钙在血液中的含量呢, 我选了B, 其实是对这道题基本上没搞明白. 请大牛帮忙详细解释一下!

你确定答案吗？

我觉得第一题选C。

C 说所有的树干都有一个特征，出现这个特征是因为雨量的不同，也就是说，出现这个特征的时间是在同一时段。根据特征年轮以后的年轮的数目，可以推断出每棵树是在特征年轮的第几年后被砍的，又由于坟墓用的是新树，就可以判断出坟墓是哪年建的，因此坟墓的相对年龄就知道了。

说白了， 就是加了个零点。

第二题很好理解啊。

一个管上升，一个管下降. 第一次你只把上升的去掉，第二次你把两个都去掉，第一次下降得更快是可以理解的。

是C, 我搞错了, 对不起, 但你是指出现那个12年的年轮的年代是已经知道的对吗? 所以根据距离多远可以判断建设古墓的建设时间, 要不怎么知道距离现代的时间呢?

第二题明白了, 先多谢了!!!!!!

不需知道绝对时间（距离现在的时间），题目只问相对时间。

读题：

establish the relative ages of the tombs