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问题 单项选择题

通过在SC关系中的 ______ 上创建惟一索引来提高查询速度。

A．(S#，C#)

B．S#

C．C#

D．GRADE

答案

参考答案：A

解析：[评析] 建立索引是加快查询速度的有效手段，在SC表中建立S#和C#的索引可以加快查询速度。

单项选择题

A glue that holds the whole Internet together is the network layer protocol, (1) .Unlike most older network layer protocols, it was designed from the beginning with internetworking in mind. Its job is to provide a (2) way to transport data grams from source to destination, without regard to whether these machines are on the same network or whether there are other networks in between them.

Communication in the Internet works as follows. The (3) layer takes data streams and breaks them up into data grams. Each datagram is transmitted through the Internet, possibly being fragmented into smaller units as it goes. When all the pieces finally get to the destination machine, they are reassembled by the (4) layer into the original datagram. This datagram is then handed to the transport layer, which inserts it into the receiving process’ input stream.

An IP datagram consists of a header part and a text part. The header has a (5) part and a variable length optional part.

3（）

A．data link

B．transport

C．network

D．application
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选择题

情景交际我最棒，根据情景选择最佳答案。

1. A letter is __________ the table. [ ]

A. in              

B. on              

C. under

2. —Where is Xiaoyong? 

    —__________[ ]

A. He's on the box    

B. He is on the bed.

3. —What is this?   

    — __________.[ ]

A. It's on the box.    

B. It's a box.

4. He is swimming __________ the lake. [ ]

A. on              

B. in                

C. under

5. —__________?

    —It's on the chair.[ ]

A.  What is it    

B.  Where is the apple  

C.  How is the apple

6. She often  __________ in the park. [ ]

A. flies kites          

B. swims              

C. walks
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