Solution:A public good is a good possessing two characteristics:a) Once the good is provided it is difficult to exclude or prevent others from consuming the good, even if they do not pay for it.
b) Consumption of the good by one individual does not reduce the amount available for others to con-sume.
Clean air and national defense both satisfy the char-acteristics of a public good. Once these goods are made available to one person, it is not possible to prevent another from consuming the goods.
A loaf of bread (Food items) would not be classified as a public good. If an individual is unwilling to pay the bakery for a loaf of bread, the bakery can prevent the individual from consuming it. Further, if a few slices of a loaf of bread are consumed, the quanti-ty of bread that is available for others to consume is decreased.