A. to study B. to be studied

C. to have studied D. to be studying

【答案】C

【解析】句意：据说林峰曾去国外留学，但我不知道他曾在哪个国家学习过。表示“据说”的be said后跟动词不定式；且根据which country he studied in可知动词不定式中的动词study发生在谓语动词之前，故用动词不定式的完成式to have +过去分词，故C项正确。