——Do you agree with what I said?
    ——I couldn't agree with you ______, that's really a good idea.
 
【正确答案】 C
【答案解析】 根据“that's really a good idea”可知回答者持赞同态度，排除D选项。“I couldn't agree with you more”表示“我非常赞同”，符合题意，故选C项。