A graph of price, P, versus quantity, Q, shows two parallel supply curves, S 1 and S 2, and a demand curve, D. The supply curves are straight lines ascending to the right, and the demand curve is a straight line descending from upper left to lower right. S 1 is to the right of S 2. S 1 intersects Curve D at point (120, 9), and S 2 intersects Curve D at point (105, 12). Point (105, 8), is marked on S 1. What is the producer share of the tax rate per unit?
a. $3
b. $2
c. $1
d. $4