Not until he met with the difficulty the importance of group work.
A. he realized
B. did he realize
C. realized he
D. he didn’t realize
网友回答
A. he realized
(答案→)B. did he realize
C. realized he
D. he didn’t realize
解析：考查倒装结构。句中否定词Not until放在句首，主句的主谓要用倒装结构，故选B。