After a 5-year absence, we returned to our mother school, and not until then it was such a beautiful school.
A. we realized
B. had we realized
C. we had realized
D. did we realize
网友回答
A. we realized
B. had we realized
C. we had realized
(答案→)D. did we realize
解析：考查部分倒装。not until放在句首时，后面句子要部分倒装，排除A,C；根据then，可知要用一般过去时态，所以选D。