Thousands of years ago people discovered the phenomenon by chance, and this was the idea of the pass first came from.
A. how
B. what
C. where
D. that
网友回答
A. how
B. what
(答案→)C. where
D. that
解析：考查定语从句。句中the phenomenon为下文the idea of the pass first came from的逻辑地点状语，故选C，where做from逻辑宾语，指代前文的the phenomenon。