Two \(5000-\mathrm{kg}\) freight cars roll without friction (one at 1 \(\mathrm{m} / \mathrm{s}\), the other at \(2 \mathrm{~m} / \mathrm{s}\) ) toward one another on a level' track. They collide, couple, and roll away together with a combined momentum of
A. zero \(\mathrm{m} / \mathrm{s}\) B. \(5000-\mathrm{kg} \mathrm{m} / \mathrm{s}\) C. \(10,000 \mathrm{~kg}\) D. \(15,000 \mathrm{~kg} \mathrm{~m} / \mathrm{s}\).
