In the graph shown here, the upward$-$sloping light gray line is the initial supply curve. The upward$-$sloping dark red line is the new supply curve. This graph shows the effect of $a$ :

A$.$ $$2$ per unit tax on sellers

B$.$ $$2$ per unit tax on buyers

C$.$ $$2$ per unit subsidy to sellers

D$.$ $$8$ per unit tax on sellers