The Following Questions Refer to the Table Below, Which Compares $\quad$

The following questions refer to the table below, which compares the % sequence homology of four different parts (2 introns and 2 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 that 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.
$\quad$$\quad$$\quad$$\quad$$\quad$$\quad$$\quad$$\text { \%/ Sequence Homology }$
$\begin{array} { l l l l l } \text { Species } & \text { Intron I } & \text { Exon I } & \text { Intron VI } & \text { Exon V } \\\text { A } & 100 \% & 100 \% & 100 \% & 100 \% \\\text { B } & 98 \% & 99 \% & 82 \% & 96 \% \\\text { C } & 98 \% & 99 \% & 89 \% & 96 \% \\\mathrm { D } & 99 \% & 99 \% & 92 \% & 97 \% \\\mathrm { E } & 98 \% & 99 \% & 80 \% & 94 \%\end{array}$
-Which of these 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, is more stable.