Definition of a reachable ancestor (Skiena TADM 2nd ed section 5.9.2)
A small modification makes the definition agree with Skiena's algorithm. 
"Let reachable_ancestor[v] denote the earliest reachable ancestor of vertex v, meaning the oldest ancestor of v that we can reach from a descendant of v by using at most one back edge."

If we view undirected edges as "two-way" directed edges, then to reach A from C would require two back edges: DB and BA.  Thus the earliest ancestor that can be reached from C by using at most one back edge is B.