The trick is that two of the three slits are blocked by obstructions that can only be seen by looking in a mirror that reflects a backward view of what's inside the box.

问题难在这三道叉中有两道是被堵着的，只能通过镜子看盒子里面的倒影才能看出来。

youdao

For longer paths, detailed path surveys may be required to determine that there are no topographic or man-made obstructions blocking the line of sight between the two facilities.

对于长距离路径，则必须仔细勘查两个设施问的路径，确保在视距以内没有地形上或人为的障碍物。

youdao

Two or three weeks later they could not only walk but could also dodge round obstructions and run upstairs.

ECONOMIST: Spinal injury: Running repairs | The