Comparison of the growth rings of ancient trees enables scientists

to determine from a piece of timber the year in which the tree used

for the timber was felled.  Hence, by analyzing the growth rings in

timber surviving from ancient buildings, archaeologists can deter-

mine precisely when those buildings were constructed.

 Which of the following is an assumption on which the argument

depends?

       The timber used for construction purposes in ancient times

        was made from very old trees.  

      The timber that was used in ancient building construction had

        not, prior to being used in the construction of the buildings,

        lain unused for an indeterminable amount of time.  

      The growth rings of any tree felled in a given year are iden-

        tical to the growth rings of any other tree felled in that year.

      The oldest of the ancient buildings that survive to the present

        day were constructed of the most durable woods.  

      Ancient builders did not use more than one type of wood for

        the construction of a given building. 

答案：A