B did we realize
因为这是个only位于句首引导的倒装句
在句首的only如果后面跟了副词、介词短语或从句等状语,则主句用倒装结构如：Only in this way can you learn English well． 【注意】：only和所修饰的状语如果不在句首,则不倒装.如：The boy was helped out only when his father arrived in a hurry．