Why is national defense a public good?

I. It is expensive to produce.

II. People who don't pay for national defense still benefit from having it.

III. One person's benefit from national defense doesn't reduce anyone else's benefit from it.

IV. It is provided by the government.

Select one:

a.II, III, and IV only

b.I, II, and III only

c.II and III only

d.I and IV only