Two \(5000-\mathrm{kg}\) freight cars roll without friction (one at \(1 \mathrm{~m} / \mathrm{s}\), the other it \(2 \mathrm{~m} / \mathrm{s}\) ) toward one another on a level track. They collide, couple, and roll away together with a combined momentum of ____________

A. zero
B. \(5000 \mathrm{~kg} \mathrm{~m} / \mathrm{s}\)  
C. \(10.000 \mathrm{kgm} / \mathrm{s}\)
D. \(15.000 \mathrm{~kg} \mathrm{~m} / \mathrm{s}\)

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Correct Answer: B