Norman Company pays a sales commission of $4\%$ on each unit sold. If a graph is prepared, with the vertical axis representing per$-$unit cost and the horizontal axis representing units sold, how would a line that depicts sales commissions be drawn?

A$.$ As a horizontal line.

B$.$ As a straight diagonal line, sloping downward to the right.

C$.$ As a curvilinear line.

D$.$ As a vertical line.

E$.$ As a straight diagonal line, sloping upward to the right.