28. It might have saved me some trouble ______ the schedule.
A
did I know
B
have I known
C
do I know
D
had I known
句意：如果我早知道日程安排，可能会省去我一些麻烦。根据题干中might have saved可知主句是对过去的虚拟，从句应用if I had known。if虚拟条件句中含有had时，可将if省去，同时将had提前构成倒装结构。故选D项。
分析句子结构可知主句是对过去的虚拟。
对虚拟语气了解不够准确。