From time to time a crocodile has snatched someone ______ he or she is even aware that the crocodile is there.
A.whenB.untilC.sinceD.before在线课程D
考查时间状语从句。句意“有时候，在人们意识到有鳄鱼之前，鳄鱼会突然捉住他。”A“当”；B“直到”；C“自从”；D“在......之前”。故选D。