焊接时，不与氮气发生作用的金属，即不能溶解氮又不形成氮化物的金属，可用N作为保护气体，这种金属是（）。 ["铜","铝","镍","铁"]
Which two of these are used in the selection of a root bridge in a network utilizing Spanning Tree Protocol IEEE 802.1D？（） [" Designated Root Cost"," bridge ID priority"," max age"," bridge ID MAC address"," Designated Root Priority"," forward delay"]
焊接区内的气体主要来源于（）。 ["焊接材料","母材","焊条药皮","空气"]
胶带内钢条折断和胶带面破损的原因有哪些？
《服务质量规范》规定：重点旅客：指老、幼、病、残、孕旅客。特殊重点旅客是指依靠辅助器具才能行动等需特殊照顾的重点旅客。（）
某种钢材焊接时高温出现了结晶裂纹，裂纹具有氧化色彩，试分析该裂纹产生机理以及防治措施
