Explain why each of the following options correctly or incorrectly completes the sentence:

"A market economy does not provide a sufficient amount of a public good like national defense because __________"

option a. private firms will provide national defense at a greater cost than the government.

option b. national defense does not sufficiently benefit individuals in society.

option c. it is impossible to withhold national defense from persons who do not pay for it.

option d. consumers are poorly informed about the benefits of national defense.