Answer
*public good: not excludable, not rival. Ex. national defense
* Private market cannot provide this good on its own.
Work Step by Step
*NOT EXCLUDABLE: Because no citizen can be prevented from using the protection of national defense.
NOT RIVAL:Because the use of this good by one person does not diminish the benefit others can get from the good.
*Private market cannot provide good on its own because nobody would pay for the good, since they cannot be excluded because they don't pay.
