Denée asks if this helps answer the question she'd posed about whether they'd end up the same way.

Which of Toby's responses to Denée's proposal that the two situations—if the collision is elastic—yield the same result is correct?

A. Toby: "You are incorrect, Denée, because when the 2-kg cart hits the 1-kg cart at rest, both move at different speeds in the same direction the 2-kg cart was originally moving, but when the 1-kg cart hits the 2-kg cart at rest, it bounces backward."
B. Toby: "You are correct, Denée, because the final momentum in both cases is the same as the initial momentum."
C. Toby: "You are incorrect, Denée, because the kinetic energy is conserved in one collision, when the 1-kg cart collides with the 2-kg cart at rest, but not in the other collision."
D. Toby: "You are correct, Denée, because the two masses trade respective final momenta after the two collisions; when the 1-kg cart hits the 2-kg cart, it slows down, and when the 2-kg cart hits the 1-kg cart, it slows down."