Question

If two balls collide with each other, they will move apart at the same speed if

Answer 1

A.
if you have seen a newton's cradle this will make sense.

in order for both of them to travel at the same speed, the balls need to have the same mass and the speed to begin with tocontinue to travel at the same speed because mass can affect the impact of the force on the balls by each other, causing each ball to have different speeds.

Answer 2

Answer a. would be correct. Since momentum is conserved, Δmv must be constant.