He left early on the ______ that he had a bad toothache and had to see the dentist.
 
【正确答案】 B
【答案解析】 instruction“指示”;excuse“托词,借口”;knowledge“知识,学问”;precondition“前提”。根据句中left early提供的语境,a bad toothache是借口,故答案为B。