Refer to Exhibit 31-2. This graph represents a positive externality situation. Given this,
which of the two curves, X or Y, represents marginal social benefits and why?
Exhibit 31-2
a. Curve X, because if there is a positive externality, negative external benefits are
associated with it: social costs external benefits – private benefits, therefore the
marginal social benefit curve must lie below the marginal private benefit curve.
b. Curve X, because if there is a positive externality, external benefits are associated
with it: social benefits = external benefits + private benefits, therefore the marginal
social benefit curve must lie below the marginal private benefit curve.
c. Curve Y, because if there is a positive externality, external costs are associated with
it: social benefits = external costs + private benefits, therefore the marginal social
benefit curve must lie above the marginal private benefit cost curve.
d. Curve Y, because if there is a positive externality, external benefits are associated
with it: social benefits = external benefits + private benefits, therefore the marginal
social benefit curve must lie above the marginal private benefit curve.
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