[单项选择]Because he could not find the fight theme ______ , his explanation was vague. A. to center his thoughts about B. in which to center his thoughts C. around which his thoughts to center D. on which to center his thoughts
参考答案:D 答案解析:此句是定语从句的省略形式,完整形式是“on which he could center his thoughts”,因此选 D。