your advice, I would have had no trouble with my papers.
A. Had I followed
B. If I followed
C. If I have followed
D. Once I followed
网友回答
(答案→)A. Had I followed
B. If I followed
C. If I have followed
D. Once I followed
解析：根据would have had可知, 这是是一个表示与过去事实相反的假设。if条件从句用过去完成时had done，又知if条件句中如有were, should, had，可以省去if，并使用倒装语序, 即were, should, had+主语+谓语动词。故答案为A。