d) [4 pts] Two astronauts in space, George and Sandra, are drifting horizontally toward each other. The moment after they collide and hold on (i.e stick) to each other, they are both at rest. Which of the following MUST be true? Circle ALL that apply... A) Sandra and George must have the same mass. B) George and Sandra must have been moving at the same speed before colliding. C) The center of mass of the combined George-Sandra system does not move. D) Before the collision, the momentum of George plus the momentum of Sandra equals Zero