Objects X and Y are connected by a string of negligible mass and suspended vertically over a pulley of negligible mass, creating an Atwood's machine, as shown in the figure. The objects are initially at rest, and the mass of object Y is greater than the mass of object X. As object Y falls, how does the kinetic energy of the center of mass of the two- object system change? Justify your selection. All frictional forces are considered to be negligible. A (B D The kinetic energy increases because the gravitational force due to Earth does positive net work on the system. The kinetic energy increases because the gravitational force due to Earth does negative net work on the system. The kinetic energy decreases because the gravitational force due to Earth does positive net work on the system. The kinetic energy decreases because the gravitational force due to Earth does negative net work on the system. Object Y Object X