Few people paid full attention to their health conditions they were seriously ill. A. whe

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A. when
B. if
(答案→)C. until
D. after
解析：考查连词辨析。A当…时；B如果；C直到…才…；D在…后；本句中的not…until…直到…才…；如果前面的句子使用的短暂性动词，就使用否定句；如果前面的动词是延续性动词就使用until来引导；句意：直到他们生了重病以后他们才会注意到他们的健康状况。故C正确。