**QUESTION**

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?

**OPTIONS**

[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 artefacts.

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

Explanation:

Archaeologists use the growth rings to determine the relative ages of the tombs, made from freshly cut logs of the same type and place (and, thus, all with the same amount of annual rainfall). The number of rings = the number of years, and width of a tree’s yearly ring depends on how much rain fell that year. Now, how does all of this help determine which tombs are older? Well, if there were no similarity between the rings on the logs of one tomb and those on the logs of another, it wouldn’t help at all. You can only determine the relative age of the logs (and thus the tombs) if there’s some overlap to the rings, so that the ring patterns can be compared. Thus, (C) is correct: If every log has the same pattern of 12 rings, then their comparative ages can be determined by how many more rings each log has after the crucial pattern of 12. (Example: A tree with 50 rings after the pattern was cut down 30 years later than a tree with 20 rings after the pattern.)

(A) tells us that “artifacts” were preserved—which needn’t, of course, be logs. Moreover, we’ve no idea what effect seepage would have on the logs’ rings, so (A) is of no help.

(B), which speaks of trees outside the valley, is irrelevant; the archaeologists only compare the rings on trees within the one valley.

(D) gives the extreme dates for the trees used, but provides no clue as to how the archaeologists determined these ages or the relative ages of the other trees.

(E) The age of the artifacts doesn’t indicate the age of the tombs; plenty of new buildings contain old items.

Previous QuestionNext Question