问题补充：
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
答案：
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， only + 状语位于句首句子要用部分倒装。
