“I bought this shirt for 35 yuan yesterday.”

“It’s on sale today for only 29. You should have waited.”

“Oh really? But how [空格] I know?”

  [A] would       [B] can       [C] did       [D] do

本题是1993年英语专业四级考试的一道真题，答案有的选择A项，有的选择C项。还有的对A项进行了改动，变成了could。我感觉选A，又说不明白。所以来请教。谢谢！