关于过去分词和形容词The spokesman was seated in front of all the journ

关于过去分词和形容词
The spokesman was seated in front of all the journalists at the press conference,____to answer all kinds of questiongs.
A.prepared B.preparing C.to be prepared D.having prepared
正确答案是A.
可是如果这里prepared是形容词的话,be prepared to do 是愿意做某事.
而D的话,表示spokesman已经准备好回答问题了.
为什么选A不选D呢?
额……我查了，be prepared to do 是准备好要做某事。
可是A和D的区别是什么我还是不理解。