A、B、C、D、E五种不同的商品要在货架上排成一排，其中A、B两种商品必须排在一起，而C、D两种商品不能排在一起，则不同的排法共有______种．

It was not until the entrance exams were in sight that ____ that I had so many books to cover

and so many exercises to finish. [ ]

A. I realized

B. did I realize

C. had I realized

D. I had realized