各种专升本 选择专业有哪些方向 ？

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甲对乙负有债务100万元，约定分别于2019年1月1日、3月1日、5月1日归还20万元、30万元、50万元，分期偿还，由丙提供保证责任，未约定保证方式与保证期间。后甲与乙协商将每笔债务均顺延2个月，乙表示同意，但未通知丙。乙于2019年1o月1 Et请求丙承担保证责任。下列说法错误的是：( ）

A.因未约定保证方式，丙承担一般保证责任
B.丙保证担保的债权为80万元
C.丙保证担保的债权为50万元
D.因延长还款期限对主债务进行了实质性变更，且未经保证人同意，丙可不再承担保证责任

Public goods are those commodities from whose enjoyment nobody can be effectively excluded.Everybody is free to enjoy the benefits of these commodities,and one person′s utilization does not?reduce the possibilities of anybody else′s enjoying the same good.
Examples of public goods are not as rare as one might expect.A flood control dam is a public?goods.Once the dam is built,all persons living in the area will benefit--irrespective of their own?contribution to the construction cost of the dam.The same holds tree for highway signs or aids to navigation.Once a lighthouse is built,no ship of any nationality can be effectively excluded from the utilization?of the lighthouse for navigational purposes.National defense is another example.Even a person who voted?against military expenditures or did not pay any taxes will benefit from the protection afforded.
It is no easy task to determine the social costs and social benefits associated with public goods.There is no practicable way of charging drivers for looking at highway signs,sailors for watching a?lighthouse,and citizens for the security provided to them through national defense.Because the market does not provide the necessary signals,economic analysis has to be substituted for the impersonal?judgement of the marketplace.

According to the passage,finding out the social costs of a public good is a__________.

A.difficult procedure
B.daily administrative duty
C.matter of personal judgement
D.citizen's responsibility