________ the building did he realize that there was danger everywhere in it.
句意：只在进入大楼后，他才意识到危机四伏。从句子结构上看，本句为倒装句，only如果后面跟了副词、介词短语或从句等状语，则主句用倒装结构。因此，本题的正确答案为C。