在OSI七个层次的基础上，划分为了（）级别。 ["一种","两种","三种","四种"]
简述斜管虫病的症状和病理变化。
In a vacuum boiling evaporator, the fresh water generated is pumped out by （）. ["a brine pump","an air ejector","a feed water pump","a condensate pum"]
批发商在销售渠道中起着无可替代的作用，有的批发商并不取得商品的所有权，而是拥有商品的转移权和谈判权，这类批发商是（）。 ["经销批发商","制造商的分销机构","专业批发商","代理批发商"]
With a vacuum boiling evaporator, the remainder of sea water fed to the evaporator is pumped out by （）. ["a brine pump","an air ejector","a feed pump","distillate pum"]
简述斜管虫病的病源。
