An expression is given by $x^{2}-18 x+n$. What value of $n$ would make the expression a perfect square trinomial, and what would the expression be written as the square of a binomial? $n=81$ and $(x+9)^{2}$ $n=9$ and $(x-9)^{2}$ $n=9$ and $(x+9)^{2}$ $n=81$ and $(x-9)^{2}$