Question

Not until they left school ______ how much their teachers loved them and helped them

1. A.
   they realized
2. B.
   had they realized
3. C.
   they would realize
4. D.
   did they realize

Answer 1

D
本题考查倒装结构。Not until放在句首，主句要发生主谓倒装，再根据时态，“他们意识到”是在“他们离开学校之后”，故用一般过去时。故选D。