单项选择题
A．ensure the continuous supply, of public goods B．reduce the difference between the rich and the poor C．get extra money to support public goods D．remind people of their duty
A．what role the market system plays in producing public goods B．how weak the market system is in producing public goods C．why some people refuse to pay for public goods D．what we can do to make use of the market system
A．keeps himself from using its benefits B．cannot enjoy its benefits at any time C．fails to keep its benefits from being used D．enjoy its benefits all the same