The reason he didn’t go to school yesterday was he was ill．
A. why；that
B. that；because
C. that；that
D. for which；what
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(答案→)A. why；that
B. that；because
C. that；that
D. for which；what
解析：考察定语从句关系词用法，根据前置词reason原因可知此处关系词应用why，为什么。当句子有几个关系词同时出现时，后面关系词一般只使用that，故选A。