下列哪些特征是管胞所特有的？（） a.细胞壁加厚 b.细胞两端尖斜 c.可输送水分和无机盐 d.是死细胞，其细胞核和细胞质消失 e.具壁孔 f.端壁打通 ["a，c，d，e，f","a，b，c，d，f","b，c，d，e，f","a，b，c，d，e"]
污泥浓缩的目的在于缩小污泥的体积，以（）混凝剂的投加量与脱水设备。 ["减小","增大","保护","维护"]
Water allowed to accumulate in the hydraulic system oil will cause an oxidation process producing by- products which are normally （）. ["gums, acids, and varnishes","removed by cellulose type filters","neutralized by oil additives","not removed by absorbent filter"]
简述智能高频开关电源的安全操作规程。
何为泛水？试述卷材屋面泛水的基本做法？
简述会展项目管理的过程。
