The professor walked onto the platform and seated himself in a chair,___ for answering questions .
A．had prepared B．being prepared C．preparing
D．prepared
选D C怎么不对？不是伴随吗？

有前半句：The professor walked onto the platform and seated himself in a chair我们知道教授已经做好了回答问题的准备，而且回答问题这个状态会持续到问题被问完，be prepared for sth : 对某事做好准备，表示状态的持续，prepare for: 为某事做准备，这个表示准备的动作，如果把这个句子变成简单句应该是：
The professor walked onto the platform and seated himself in a chair and he was prepared for answering question.因为前后主语一致，所以变成：
The professor walked onto the platform and seated himself in a chair ，prepared for anwsering the questions.而preparing for anwsering the questions 则是表示准备的动作，很显然这个句子表示的是回答问题这个状态的持续，而不是短暂性的准备动作，所以答案选prepared