设S为学生关系，Sc为学生选课关系，Sno为学生号，Cno为课程号，执行下面SQL语句查询结果是(30)。Select S. * From S，SC Where S. Sno=Sc. Sno and Sc. Cno='c2'

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In the following essay, each blank has four choices. Choose the best answer and write down on the answer sheet.

 In security, an(71)is a form. of possible loss or harm in a computing system; examples of exposures are unauthorized disclosure of data, modification of data, or(72)of legitimate access to computing. A vulnerability is a weakness in the security system that might be exploited to cause loss or harm. A human who exploits a vulnerability perpetrates an attack on the system. Threats to computing systems are circumstances that have the potential to cause loss or harm; human attacks are examples of threats, as are natural disasters, inadvertent human errors, and internal hardware or software(73). Finally, a control is a protective measure an action, a device, a procedure, or a technique - that reduces a vulnerability.  The major(74)of computing systems are hardware, software, and data. There are four kinds of threats to the security of a computing system: interruption, interception, modification, and(75). The four threats all exploit vulnerabilities of the assets in computing systems.

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In the following essay, each blank has four choices. Choose the best answer and write down on the answer sheet.It is more and more usual that users of Information Retrieval(IR)systems must face up to response of hundreds or thousands of(66)documents. A significant proportion of them are moreover(67)or redundant. This is a straight consequence of two well - known facts: the incessant growing of documental databases and the lack of ability of most users to define(68)information needs. In such a framework, further effective retrieval techniques are only part of the solution. Interfaces with alternative ways for organizing and(69)the results returned by a(70)would be of great utility assisting users to find relevant documents. Document clustering may play a central role as an effective technique for organizing query results.