We know that the archaeologists were successful in determining the relative ages of ancient tombs at Pazyryk and that those who constructed the tombs both used freshly cut logs and were required by tradition to use only trees which grew in the Pazyryk Valley.
The issue is that the rings in a tree's trunk only tell you how old the tree was when it was cut. The rings do not tell you when the tree was cut nor when it first sprouted.
This is why answer choice (C) is correct. It provides the archaeologists with a marker in time by which to assess the tree's rings. Determining when this pattern of weather occurred would allow the archaeologists to assess when the tomb was built by counting the number of rings following this weather pattern.
Answer choice (B) is incorrect because it doesn't provide us with an explanation. We already know that those who constructed the tombs were required by tradition to use only trees which grew in the Valley. While these distinct patterns of rings would help to confirm this, they do not provide us with an objective mark in time by which to determine the age of the tombs.
Hope this helps! Please let us know if you have any further questions.
