Consider a Phylogenetic Tree

Consider a phylogenetic tree. The node at the bottom of the tree (i.e., the "root") would represent which of the following?

A) the common ancestor of all organisms included in the tree
B) the common ancestor of a subset of the organisms included in the tree
C) an organism that is unrelated to all other organisms included in the tree
D) an organism that is the common ancestor for organisms not included in the tree
E) Nothing; the node serves only as the starting point of the tree.