6/7 please help.

6. Two objects collide on a horizontal frictionless surface. Which of the following statemts MUST be true. The objects stick together Momentum is conserved Kinetic energy is conserved O We can't determine if kinetic energy or momentum are conserved without more information 7. An empty coal mining cart of mass M coasts along a track at 3.00 m/s until it attaches to a stationary cart of mass 2M. The final speed of the two carts immediately after the collision is 0.667 m/s 1.00 m/s 1.50 m/s 2.00 m/s