he has been waiting for the letter.which = the letter
for which引导了定语从句;而B中缺少关系代词的which,无法构成定语从句;C中的it不是关系代词;D中的关系代词whom表示人,而先行词的letter不是人,是事物
he has been waiting for the letter.which = the letter
for which引导了定语从句;而B中缺少关系代词的which,无法构成定语从句;C中的it不是关系代词;D中的关系代词whom表示人,而先行词的letter不是人,是事物