Which is the shortest path?
A person is located at point D in a river. He needs to reach the river bank AB to pick up something, leave it at the bank BC, and then return to his original position.
1. How to construct his shortest path?
(Hint: Which is the shortest path between two points on a plane?)
2. Try to move point A, B, or C, such that the red line disappears. Which is the shortest path for this scenario?