For a Weil divisor $D$ on a normal variety $X$, the sheaf $\mathcal{O}_{X}(D)$ associated to $D$ is defined by $$ \Gamma\left(V, \mathcal{O}_{X}(D)\right):=\left\{f \in \mathbb{C}(X)^{*}|(f)|_{V}+\left.D\right|_{V} \geq 0\right\} \cup\{0\} $$ for each open subset $V \subset X$, where $(f)$ is the principal divisor given by a rational function $f \in \mathbb{C}(X)^{*}$.
I know it is reflexive. I wonder how to show $\mathcal{O}_{X}(D)$ is a rank 1 sheaf (this is called divisoral sheaf). Notice that by Hartshorne Ex 6.10 (p148), we need to show
$$\text{ }\mathcal{O}_X(D)_\eta\simeq \mathcal{O}_{X, \eta}\simeq K(X)$$
for the generic point $\eta$ of $X$. Here, $K(X)$ denotes the function field.
I can't see how to show this equality above.
Many thanks.
X is smooth outside a codimension $\geq 2$ locus $Z$, so $\mathcal{O}(D)$ is just an invertible sheaf on $X-Z$ and has rank 1.