【名词&注释】
工作方式(working mode)、水分离器(water separator)、能见度不良(restricted visibility)、工作中的
[单选题]When anaerobic bacteria convert the sewage to an inert sludge, .
  A. no oxygen is needed 
  B. oxygen is necessary 
  C. oxygen is given off 
  D. no gas is produced 
 
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