请教一道解释题

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 trees grwoing 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?

答案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.

题目中说年轮可以确定树的年龄。年轮的粗细代表当年不同数量的雨季。然后人们成功的确定了古墓的相对年龄。C答案中，只表明:12个年轮代表6年干旱，3年雨季，又是3年干旱。这个怎么测定年龄了？

这道题意思是通过 树上的年轮 判断古墓的相对年龄，也就是谁前谁后，那么参照的就是那个特殊的12年，因为所有的树都经历了那特殊的12年，所以必然会在年轮上有所体现，而通过观察 那12年的年轮的在每棵树的不同位置就能判定相对年龄了。（比如 有的树那12年的年轮更靠中心，而另一些树木那12年的年轮靠边上，通过相对位置判断相对年龄）

不知道我解释清楚没