一道英语题！！麻烦解释一下！！
29. That was really a splendid evening. It’s years ______ I enjoyed myself so much.
A. when B. that C. before D. since
为什么是D不是C？
参考答案:选择before主句时态不对;it's /has been years since +主语+did
it will be years before you attend the college entrance examination./it would be years before i enjoyed myself so much.
明白吗