Ten very small rubber balls begin equally spaced inside a 55\:m long tube. They instantly begin to roll inside the tube at a constant velocity of 1\:m/s. When a ball reaches an end of the tube, it falls out of the tube. When two balls bump into each other, they both instantly reverse directions but continue to roll at 1\:m/s. Five con gurations giving the initial direction of movement of each ball are shown. All gaps indicated in the diagram are the same length and are equal in length to the distance from the ends of the tube to the nearest ball. For which con guration will it take the least time for more than half of the balls to fall out of the tube?
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