Problem: An astronaut in a space suit is motionless in outer space. The propulsion unit strapped to her back ejects some gas with velocity of magnitude 40.0 m/s. The astronaut recoils with a velocity of 1.00 m/s in the opposite direction. If the mass of the astronaut and space suit after the gas is ejected is 100 kg and the mass of the gas ejected is 2.50 kg, then the total kinetic energy is
A: 4,150 J.
B: 3,430 J.
C: 2,050 J.
D: 5,070 J.