In order to find the missing boy the young couple all they can over the past twenty-four

问题补充：

In order to find the missing boy, the young couple all they can over the past twenty-four hours.

A. do

B. did

C. have been doing

D. had done

答案：

A. do

B. did

(答案→)C. have been doing

D. had done

解析：从时间状语over the past twenty-four hours可以得知用完成时，但找的动作还很有可能继续下去，故使用现在完成进行时。句意：为了找到那个失踪的男孩，那对夫妇在过去24个小时里尽一切所能在找。