It was announced that only when the examination papers were handed in to go out of the classroom．
A. the students would be permitted
B. had the students been permitted
C. would the students be permitted
D. the students had been permitted
网友回答
A. the students would be permitted
B. had the students been permitted
(答案→)C. would the students be permitted
D. the students had been permitted
解析：考查倒装结构。句中主语从句用only + 状语从句 + 倒装结构，根据句意选C。