It was when searching for the lost book whose cover was missing ______ a secret letter addressed to his mother.A.what he foundB.until he foundC.that he foundD.he had found
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C解析考察强调句型。本题是一个强调句型，强调的是时间状语when searching for the lost book whose cover was missing，强调句型的后半句使用陈述语序。故C正确。