Use Figure: Externalities and Deadweight Loss I

Use Figure: Externalities and Deadweight Loss I. The figure shows the marginal private benefit, marginal social benefit, and marginal private cost for a good. Based on the figure, the marginal external benefit from purchasing the good is:
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Figure: Externalities and Deadweight Loss I

A) $5.
B) $10.
C) $15.
D) $20.