(A) doesn't mention the logs at all, so it's not relevant to explaining our confusion. The contents of the tombs don't matter, just how archaeologists used logs to date those tombs.

The stimulus tell us that these tombs only used Pazyryk Valley logs, so a comparison with other logs doesn't help us. Even if Pazyryk Valley trees have a distinct ring pattern, how would that tell the archeologists when each tomb was constructed?

If the stimulus was concerned with the source of the logs, then (B) could be a strong answer choice. But since the confusion we're trying to resolve is about the dates when different logs were cut, knowing how Pazyryk Valley trees are different from other trees isn't useful.

(C) gives us a common point of comparison between all the logs: all of them lived through this distinctive drought-rain-drought period. This explains how the archaeologists could date the tombs, by comparing where in each log's rings this sequence took place. They could determine if one tomb's logs were cut right after this time period, but another's were only cut 50 years later.

(D) doesn't help us because we already know that it's possible to determine a tree's age. What we're missing is an explanation of how archaeologists could use that to determine the relative ages of the tombs. (D) doesn't get us any closer to that; it just adds detail to one of the stimulus' claims.

An important detail here is that the ages of the trees don't necessarily indicate when they were cut, and therefore the tombs' relative ages. A 90-year-old tree and a 450-year-old tree could be cut and used to build two different tombs years apart, or on the same day. So how can we tell which tombs are older using the logs' rings? (D) doesn't answer that.

The cultural artifacts found in the tombs aren't relevant to resolving the question of how archaeologists dated the tombs using the rings of the logs used to construct the tombs.