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 Intron I's relatively high sequence homology among these five species?

A) It is the most-upstream of this gene's introns.
B) It was once an exon, but became intronic in the common ancestor of these five species.
C) Due to alternative gene splicing, it is often treated as an exon in these five species; as an exon, it codes for an important part of a polypeptide.
D) It has a relatively high average rate of mutation.