If the Marginal External Cost of Building a Children's Playground

Multiple Choice

If the marginal external cost of building a children's playground equals zero,then the
i.marginal private cost equals the marginal social cost.
ii.marginal social cost equals zero.
iii.marginal private cost equals zero.

A) i only
B) ii only
C) iii only
D) ii and iii
E) i and ii