Use the Following Figure to Answer the Question : Figure

Multiple Choice

Use the following figure to answer the question : Figure 20-1 : D₁ and S₁ are the private demand and supply curves,of a commodity produced by a competitive firm.S₂ reflects the social marginal cost of production,while S₀ represents the external marginal cost.
-In Figure 20-1,the marginal external cost at the equilibrium output level is:

A) $0.
B) $40.
C) $10.
D) $28.