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
网友回答
A. left
B. have left
(答案→)C. had left
D. have been left
解析:考查时态: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,选C。