Laker 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?

Multiple Choice

As a straight diagonal line, sloping downward to the right

As a straight diagonal line, sloping upward to the right

As a horizontal line

As a curvilinear line

As a vertical line