在变动成本法下，息税前利润真正成了反映企业（）多少的晴雨表。 ["产量","销售量","变动成本水平","固定成本水平"]
活塞及活塞环在安装时无方向性要求。
《煤矿安全规程》规定：采掘工作面进风流中，氧气浓度不得低于（）。 ["12%","20%","20.96%"]
关于烟酸缺乏的原因正确的是（）。 ["食物中含量太低","植物性食物中不含烟酸","动物性食物中含量很低","玉米中的烟酸为结合型，不易被人体吸收"]
You are creating a Windows Forms application by using the .NET Framework 3.5 The application is used by a financial service provider.You discover that the service provider transfers large amounts of data by using XML.You need to read and validate the XMLdocuments in the most time efficient manner.Which technology should you use？（） ["The XmlReader class","The XmlDocument class","The XmlResolver class","The LINQ to XML method"]
电潜泵为什么要在停泵半小时后才能重新启动？
