A 56-kg person walks due north with a speed of 1.2 m/s, and her 7.0-kg dog runs directly toward her, moving due south, with a speed of 2.0 m/s. What is the momentum of the person-dog system after they meet?

a) 0 kg·m/s
b) 14 kg·m/s
c) 28 kg·m/s
d) 42 kg·m/s