A solid uniform disk of mass $M$ is rotating on a fixed frictionless vertical axle. A small putty block of mass $M$ is released from rest above it so that it lands and sticks half way between the disk's axle and its rim. What is the ratio of the final kinetic energy of the disk$-$putty system to its initial kinetic energy at the moment when the putty block is released?

$4/9$

$1$

$0$

$2/3$