@Sarah Chan object A has kinetic energy of 2x^2, right.And kinetic energy of object B is 2v^2 which is 72x greater than object A's kinetic energy(2x^2), so kinetic energy of object B is 2x^2+72x.Then 2v^2=2x^2+72x v^2=x^2+36x v=sqrt(x^2+36x) speed of the object B is, thus, sqrt(x^2+36x) Now, momentum of object A is 4x So, the ration of speed of object B to the momentum of object A is (sqrt(x^2+36x))/4x, which is in option B.