Not until we ______ the school for children with disabilities ______ how they overcome difficulties.
A. had visited/did we know
B. visited/had we known
C. visited/did we know
D. have visited/did we know
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Giải thích: Dùng cấu trúc đảo ngữ, cấu trúc: Not until + mệnh đề ở quá khứ đơn + did + S + V(bare)
Áp dụng cấu trúc, ta chia $visit→visited$