液压拉伸器活塞顶部设有泄放旋塞，其主要的作用是（）。
A. 在使用时，用来观察有无漏油现象
B. 用来判断密封环有无失效
C. 加压前用来泄放油中的空气
D. 压力过高时放油减压
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焊接电流越大，电弧的温度就越高，则电弧气氛中的电离程度和热发射作用就（），电弧燃烧也就越稳定。
A. 越弱
B. 越强
Which two are effects of connecting a network segment that is running 802.1D to a network segment that is running 802.1w？（）
A. the entire network switches to 802.1D and generates BPDUs to determine root bridfe status.
B. A migration delay of three seconds occurs when the port that is connected to the 802.1D bridge comes up
C. The entire network reconverges and a unique root briddge for the 802.1D segment， and a root bridge for the 802.1W segment， is chosen
D. the first hop 802.1w switch that is connected to the 802.1D runs entirely in 802.1DCompatibility mode and converts the BPDUs to either 802.1D or 802.1W segments of the network
E. Classic 802.1D timers， Such as forward Delay and Max-age， will only be used as a backup， and will not be necessary if point-to-point links and edge prots are properly identified and set by the administrator.
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