On hearing of the case some time later, Conan Doyle was convinced that the man was not guilty, and immediately went to work to
ascertain
the truth.
【正确答案】
C
【答案解析】解析：句中词“ascertain”表示“确定，探知”。A项表示“探测，探究”；B项表示“获得，得到”；C项表示“检验，查证”；D项表示“搜索，调查”。C项的意思与原文相符，所以应该选C。