问题
选择题
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
答案
答案：C
题目分析：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。