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结构安装前的准备工作有哪些？

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论述浮动抵押的概念、优点与效力。

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电动钻机的（）主要包括绞车、天车、游动滑车、大钩、水龙头、转盘、井架等。

A. 固井系统
B. 钻井系统
C. 钻井液系统
D. 控制系统

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轨道电路应能防护牵引电流的干扰，采用非工频轨道电路，与（）牵引电流区分。

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You are working in an online transaction processing （OLTP） environment. You used the FLASHBACK TABLE command to flash back the CUSTOMERS table. Before executing the FLASHBACK TABLE command， the System Change Number （SCN） was 663571. After flashing back the CUSTOMERS table， you realize that the table is not in the correct state and the resultant changes are not what you had desired. So， you need to reverse the effects of the FLASHBACK TABLE command while ensuring that： a） No other user data in the database is affected. b） The operation takes the minimum possible time. Which option would you choose？（）

A. use the ROLLBACK command with SCN 663571
B. perform Flashback Transaction Query with SCN 663571
C. execute the FLASHBACK DATABASE statement to retrieve the CUSTOMERS table as it was at SCN 663571
D. execute another FLASHBACK TABLE statement to retrieve the CUSTOMERS table as it was at SCN 663571

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