How to understand phylogenetic trees

A tree is a mathematical structure which is used to model the actual evolutionary history of a group of sequences or organisms. This actual pattern of historical relationships is the phylogeny or evolutionary tree which we try and estimate. A tree consists of nodes connected by branches (also called edges). Terminal nodes (also called leaves, OTUs [Operational Taxonomic Units], or terminal taxa) represent sequences or organisms for which we have data; they may be either extant or extinct. Internal nodes represent hypothetical ancestors; the ancestor of all the sequences that comprise the tree is the root of the tree. The nodes and branches of a tree may have various kinds of information associated with them. For example some methods of phylogeny reconstruction (e.g., parsimony) endeavour to reconstruct the characters of each hypothetical ancestor; most methods also estimate the amount of evolution that takes place between each node on the tree, which can be represent as branch lengths (or edge lengths). Trees with branch lengths are sometimes called weighted trees. Problem: What is the genetic distance between human and chimp as shown in the tree above? A) substitutions per site B) substitutions per site C) substitutions per site