The writer gives the example of national defense i

单项选择题

It would be difficult to have a society like ours without public goods such as defense, transportation, and other services.
A public good is one that a person can use without reducing the use of it for another person. One of the best examples of a public good is national defense. One person can benefit from our national defense without reducing another person"s benefits. In fact, it is difficult to keep any person in our society from enjoying the benefits of national defense.
The market system does not work well in producing public goods. This is because a person who refuses to pay for a public good cannot be kept from using it. Suppose that the neighbors in a high crime area decide to hire a police force. Each neighbor, except Mrs. Smith, agrees to pay $100 a year for it. Mrs. Smith refuses to pay because she knows that if all the others pay the $100, the police will guard the area anyway. So Mrs. Smith can enjoy the services of the police force without paying $100.
The market system has no way to deal with this type of problem. For this reason, we cannot ask each person to make a direct payment in the form of product price. Therefore, we collect money for public goods by using taxes.

The writer gives the example of national defense in paragraph 2 in order to show that ______.

A．a public good is a shared benefit for the whole society
B．all the people should be responsible for national defense
C．there will be no safety if there is no national defense
D．some public goods are more important than others