It was when searching for the lost book whose cover was missing a secret letter addressed to his mother.
A. what he found
B. until he found
C. that he found
D. he had found
网友回答
A. what he found
B. until he found
(答案→)C. that he found
D. he had found
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。