Answer:

Step-by-step explanation:
The center of mass of the system (two girls) is constant, as the velocity of the center of mass of the system is also constant.

The initial velocity of the system is zero, since both girls are at rest. So the velocity of the total system at any point should be zero as well.

This is true, because there is no friction between the girls and the ground. Otherwise, the velocity of the center of mass wouldn't be constant.