The question is from Physics book titled "Physics for Scientists and Engineers" by Randall D. Knight. Attached is part of chapter 10.1.
"Objects A and B are oblivious to our choice of system, so change in total kinetic energy for system 2 is exactly the same as for system 1."
How is that possible? System 1 has kinetic energy because there are external forces but System 2 doesn't have external forces.