a$.$ Consider the expressions ancestor$($X$,$ Y $)$ and ancestor$($dan$,$ aaron$).$ Give a unifying substitution, if there is one, or explain why they can$$t be unified if there isn$$t$.$

b$.$ Consider the expressions ancestor$($X$,$ father$($X$))$ and ancestor$($aaron$,$ dan$),$ and assume it is known that dan $=$ father$($aaron$).$ Give a unifying substitution, if there is one, or explain why they can$$t be unified if there isn$$t$.$

c$.$ Give the composition of the substitutions $\{$a$/$Z$,$ Y $/$X$\}$ and $\{$Z$/$W$,$ b$/$Y $\}$ in that order