In Diagram 1, a ball X with mass $0.15\text{ kg}$ is moving horizontally at a velocity of $2.1\text{ m/s}$ towards ball Y, which is moving in the opposite direction at $0.4\text{ m/s}$. Diagram 2 gives the velocities of the balls just after the collision. What is the mass of ball Y?
- A$0.15\text{ kg}$
- B$0.19\text{ kg}$
- C$0.35\text{ kg}$
- D$0.79\text{ kg}$