What part of a woody stem forms rings that indicate tree’s age?

cambium layer

The general rule of thumb is that one ring stands for one year, or one season of growth. A surprisingly accurate old tale, these rings are the result of cell division within the cambium layer of the woody stem.

ca = ec

cpctc makes it so that corresponding parts of the triangle you proved to be equal are equal, the other options aren't part of the part of the triangles you proved to be equal.