For each of the following pairs of expressions, either choose a most general unifier (mgu) for them or indicate that they are not unifiable. P(f(a, x, y)) and P(f(a, b, u)) [ Select ] {x→b, y→u, y→a} {x→b, y→u} {x→b, y→f(b)} {b→x, y→u} Not unifiable {x→f(f(x)), y→f(x)} {x→b, y→f(b), u→b} g(x, y, b)) and f(a, b, b) [ Select ] {x→b, y→u} {x→f(f(x)), y→f(x)} {x→b, y→f(b)} {x→b, y→f(b), u→b} No unification is possible {x→b, y→u, y→a} P(g(y, u), b, f(x)) and P(g(f(u), u), x, y) [ Select ] {x→b, y→f(b)} {x→b, y→f(b), u→b} {x→b, y→u, y→a} {x→f(f(x)), y→f(x)} No unification is possible {x→b, y→u} f(g(a), g(h(a, b)), x) and f(g(x), g(y)) [ Select ] {x→f(f(x)), y→f(x)} {x→b, y→u, y→a} {x→b, y→u} {x→b, y→f(b)} No unification is possible {x→b, y→f(b), u→b} P(x, y) and P(f(y), f(x)) [ Select ] {x→b, y→u} {x→f(f(x)), y→f(x)} {x→b, y→f(b), u→b} {x→b, y→f(b)} No unification is possible {x→b, y→u, y→a} Hint: If you can't tell at a glance, use the algorithm! Check your work by performing the substitution.多重下拉选择题