The Next Questions Refer to the Following Table, Which Compares

The next questions refer to the following table, which compares the % sequence homology of four different parts (two introns and two exons) of a gene that is found in five different eukaryotic species. Each part is numbered to indicate its distance from the promoter (e.g., Intron I is the one closest to the promoter) . The data reported for species A were obtained by comparing DNA from one member of species A to another member of species A.
% Sequence Homology

-Which of the following is the best explanation for the high degree of sequence homology observed in Exon I among these five species?

A) It is the most-upstream exon of this gene.
B) Due to alternative gene splicing, this exon is often treated as an intron.
C) It codes for a polypeptide domain that has a crucial function.
D) These five species must actually constitute a single species.
E) This exon is rich in G-C base pairs; thus, it is more stable.