Identify the correct formula for $x_A.$

Select the correct answer below:

$\frac{R_{\text{a}\text{d}\text{a}}}{P_A}$

$\frac{P_A}{x_A}$

$\frac{n_{\text{T}\text{o}\text{t}\text{e}\text{l}}}{n_A}$

$\frac{n_A}{n_{\text{T}\text{o}\text{d}\text{a}\text{l}}}$