The answer upstairs is, if the river width is not considered, it is: (for convenience of description, it is assumed that the river flows from top to bottom, A is on the left bank and B is on the right bank) Move point B to the left in the direction perpendicular to the river for a distance of river width, and record it as b'; Connect A and B', and straight line AB' has an intersection with the left bank, where the distance to build a bridge is the shortest. White and black rice?