Only one strand will be transcribed and the other servers as a coding stand. ... Without it, the single strand DNA with exposed nitrogenous bases is very unstable so two strands actually stabilise the structure.
Based on the attached Image;
The two species that are likely to have the most similar DNA base sequences
are C and D. An evolutionary tree or a phylogenetic tree, like the one shown on the image is used to indicate which ancestors gave rise to which descendants. The tree represents the evolutionary relationships among a set of organisms or groups of organisms, called taxa. The tips of the tree represents groups of descendant taxa and the nodes represents the common ancestors of these descendants.