The first possible answer is incorrect because the original value of $\mathrm{X}$ has changed before $\mathrm{Z} \leq \mathrm{X}$ is followed. Also, the operation $\mathrm{X} \leq \mathrm{Y}$ will lose the reference to $\mathrm{Y}$ (that is, nothing will be pointing at $\mathrm{Y}$, and we cannot access or change the value of $\mathrm{Y})$. The second possible answer is incorrect because it attempts to swap $\mathrm{X}$ and $\mathrm{Z}$ without temporary storage. As in the first possible answer, we lose the reference to $Y$. The third possible answer is incorrect because it misreads $\mathrm{A} \leq \mathrm{B}$ to mean "box $A$ points to box $B.$" The fourth possible answer is correct because unlike the first possible answer, it correctly uses the "temporary value $H.$"
