People in Nome all had diphtheria, a disease that spreads very fast. If ________, it would produce a powerful poison that would kill the patient.A.untreatingB.to untreatC.untreatedD.untreat
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C解析这题考查if条件句的省略，如果主从句的主语一致，可以省略从句的主语和be动词，这句话的if条件句是省略的，所以是主语a disease被省略了，a disease和untreat之间是被动关系，所以用C。