An athletics squad trains on a long straight track with dots marked at 10m intervals. The coach sets out cones on some of the dots for the squad's sprint training drills. She wants the squad to be able to run any distance which is a multiple of 10m, up to some maximum distance which depends on the number of cones. Explain why it is not possible to place four cones A, B, C, D, in a line so that each multiple of 10m up to 70m can be run between two of the cones.