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问题 单项选择题

将特定的资源用于某种用途而取得的收益是以放弃将该资源用于其他有利可图的用途为代价的，这种代价的最大预期收益就是机会成本。
根据上述定义，下列不正确的是( )。

A．农民有一块地，可养鸡、养羊、养牛，但是只能饲养一种。农民选择养牛的机会成本就是养鸡和养猪收益的总和
B．老王选择将10000元存人银行，而不是购买基金，他的机会成本就是基金所获得的最大收益
C．某公司将所拥有的经济资源用来生产自行车，而放弃用其来生产汽车，那么他的机会成本就是生产汽车所带来的最大收益
D．小张向单位请假在排队买火车票，他也是有机会成本的

答案

参考答案：A

解析： 机会成本是指将资源用作一种用途后而放弃的用作其他用途取得的最大收益。B、C均是选择了一种用途，放弃的是另外一种用途取得的最大收益的机会成本。小张向单位请假排队买票，他的机会成本就是一天的工资，也是有机会成本的。农民只能选择饲养一种，那么选择养牛的机会成本就是放弃养猪或养鸡的机会成本，而不是养鸡和养猪机会成本的总和。故选A。

单项选择题

Electronic mail has become an extremely important and popular means of communication.
The convenience and efficiency of electronic mail are threatened by the extremely rapid growth in the volume of unsolicited commercial electronic mail. Unsolicited commercial electronic mail is currently estimated to account for over half of all electronic mail traffic, up from an estimated 7 percent in 2001, and the volume continues to rise. Most of these messages are fraudulent or deceptive in one or more respects.
The receipt of unsolicited commercial electronic mail may result in costs to recipients who cannot refuse to accept such mail and who incur costs for the storage of such mail, or for the time spent accessing, reviewing, and discarding such mail, or for both. The receipt of a large number of unwanted messages also decreases the convenience of electronic mail and creates a risk that wanted electronic mail messages, both commercial and noncommercial, will be lost, overlooked, or discarded amidst the larger volume of unwanted messages, thus reducing the reliability and usefulness of electronic mail to the recipient. Some commercial electronic mail contains material that many recipients may consider vulgar or pornographic in nature.
The growth in unsolicited commercial electronic mail imposes significant monetary costs on providers of Internet access services, businesses, and educational and nonprofit institutions that carry and receive such mail, as there is a finite volume of mail that such providers, businesses, and institutions can handle without further investment in infrastructure. Many senders of unsolicited commercial electronic mail purposefully disguise the source of such mail.
Many senders of unsolicited commercial electronic mail purposefully include misleading information in the messages’ subject lines in order to induce the recipients to view the messages. While some senders of commercial electronic mail messages provide simple and reliable ways for recipients to reject (or "opt-out" of) receipt of commercial electronic mail from such senders in the future, other senders provide no such "opt-out" mechanism, or refuse to honor the requests of recipients not to receive electronic mail from such senders in the future, or both.
Many senders of bulk unsolicited commercial electronic mail use computer programs to gather large numbers of electronic mail addresses on an automated basis from Internet websites or online services where users must post their addresses in order to make full use of the website or service.
The problems associated with the rapid growth and abuse of unsolicited commercial electronic mail cannot be solved by the government alone. The development and adoption of technological approaches and the pursuit of cooperative efforts with other countries will be necessary as well.

The unwanted e-mail problem can be solved if ______.

A．the government takes action

B．a new technology is adopted

C．more people are aware of the problem

D．joint efforts are made and new technology is used
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单项选择题 A1型题

诊断病毒性肝炎最可靠的依据是（）。

A.发病季节

B.起病方式

C.症状及体征

D.接触史

E.病原学及肝功能检查结果
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