29.Who do you think would___the letter of
congratulations to the teacher?
A.have written B.have wrote
C.have to write D.have writes
答案给的A,would have written,怎么理解?

首先明确这是虚拟语气的用法。

这个架构完整的句型应是：Had done （had done 结构作条件状语从句）， would have done (would have + done 主句架构）。

如果补全或说假设没说出的头一句（在语境下是可省略不说的），那这句可这样说：

Had it been from a different person (或：Had it been written by a different person), who do you think would have written the letter of congratulations to the teacher?

其中“do you think" 只在句中作“插入语”成分，不作句子的其他任何成分。

虚拟语气的这种固定时态架构用来体现对过去已发生情况的假设性的判断。这句内含意思是本来会是谁谁写的，可这“谁谁”没写，实际情况是现在是别的人写来的贺信。