ELISA法测定肿瘤标志物，常引起误差的原因是()

A．抗体来源和纯度不同

B．钩状效应

C．标本冲洗不当

D．抗凝血

E．不同工作人员操作

Bob insisted that he_______nothing wrong and  _____  from work.  [ ]

A. did; won't be fired  

B. had done; not be fired  

C. should do;mustn't be fired  

D. do; shouldn't be fired