By the time he realizes he ______ into a trap, it'll be too late for him to do anything about it.
A.walksB.walkedC.has walkedD.had walked在线课程C
解析：
本题考查时态。此题中的by the time...意为“到……时间为止”，在句中作时间状语。又因为realizes为一般现在时，所以可以排除B、D两项。A项表示“经常性的动作”，也不合题意。故选C。