I can't possibly forget the days ———to pay a visit to your school ten years ago.
如果题目是这样的话,答案应该是D.
句子主干部分是“I can't possibly forget the days.” 而“———to pay a visit to your school ten years ago”这一部分是修饰the days的定语.如果变为主动语态,可以说spend the days paying a visit to your school ten years ago,也可以说cost the days to pay a visit to your school ten years ago.题目中是to pay而不是paying,所以应该选择D.