The graph above shows the position x as a function of time for the center of mass of a system of particles of total mass 6.0 kg. For a very short time interval around 2.0 s, an external force is exerted on an object in the system. What is the resulting change in momentum of the system? 
选项为:A:$$52$$~kg·m/s  B:$$6$$~kg·m/s  C:$$-6$$~kg·m/s  D:$$-18$$~kg·m/s  请回答其中一个选项.