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?

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.