A radioactive nucleus, initially at rest in the lab frame, undergoes alpha decay into a daughter nucleus and an alpha particle. Observer $A$ is in the lab frame; observer $B$ moves at constant nonzero velocity $\vec{u}$ relative to $A$. Each observer computes, for the decay event, the following three quantities: (I) The change in momentum of the daughter nucleus, $\Delta\vec{p}_d$. (II) The change in kinetic energy of the daughter nucleus, $\Delta K_d$. (III) The total change in kinetic energy of the system (daughter + alpha), $\Delta K_\text{tot}$. Which of these quantities must have the same value for both observers, regardless of $\vec{u}$?