【题目】The boys followed the marks that they ______, until finally they found their way back to the campsite.
A. left B. have left
C. had left D. have been left
在线课程【答案】C
【解析】
试题分析：C；考查时态。过去完成指发生在过去之前，也就是过去的过去的动作。句意：男孩跟随着他们留下的记号，直到最后他们到了回到营地的路。Leave the marks是发生在follow the marks之前的，所以用过去完成时，故选C。