电压不但有大小，还有一定的方向，通常规定负电荷运动的方向为电压的方向。
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You have an SQL query that takes one minute to execute. You use the following code segment to execute the SQL query asynchronously.IAsyncResult ar = cmd.BeginExecuteReader();You need to execute a method named Do Work() that takes one second to run while the SQL query is executing. DoWork() must run as many times as possible while the SQL query is executing. Which code segment should you use? （）
A. while（ar.AsyncWaitHandle==null）{DoWork（）；}dr=cmd.EndExecuteReader（ar）；
B. while（!ar.IsCompleted）{DoWork（）；}dr=cmd.EndExecuteReader（ar）；
C. while（Thread.CurrentThread.ThreadState==ThreadState.Running）{DoWork（）；}dr=cmd.EndExecuteReader（ar）；
D. while（!ar.AsyncWaitHandle.WaitOne（））{DoWork（）；}dr=cmd.EndExecuteReader（ar）；
国家水准网是采用从整体到局部、由（）、逐级布设、逐级加密的。按其布设密度和施测精度的不同分为一、二、三、四等
磨煤机主电机冷却水取自闭式水；
A. 正确
B. 错误
In IPv6 network, an IPv6 overlay tunnel is required to communicate with isolated IPv6 networks across the existing IPv4 infrastructurE. There are currently fiv e IPv6 overlay tunnel types. Which three IPv6 overlay tunnel statements are true？（）
A. Overlay tunnels can only be configured between border routers capable of supporting IPv4 and IPv6.
B. An manual overlay tunnel supports point - to - multipo int tunnels capable of carrying IPv6 and Connectionless Network Service (CLNS) packets.
C. Overlay tunneling encapsulates IPv6 packets in IPv4 packets for delivery across an IPv4 infrastructure.
D. Overlay tunnels can be configured between border route rs or between a border router and a host capable of supporting IPv4 and IPv6.
E. Cisco IOS supports manual, generic routing encapsulation (GRE), IPv4 - compatible, 6to4, and Intra - Site Automatic Tunnel Addressing Protocol (ISATAP) overlay tunneling mechan isms.
F. Cisco IOS supports manual, generic routing encapsulation (GRE), IPv6 - compatible, 4to6, and Multiprotocol Label Switching (MPLS) overlay tunneling mechanisms.
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