设 $I=\int \frac{x \mathrm{~d} x}{a+b x^2}$, 则 $I=$.
$\text{A.}$ $\frac{1}{2} \ln \left|a+b x^2\right|+C$.
$\text{B.}$ $\frac{b}{2} \ln \left|a+b x^2\right|+C$.
$\text{C.}$ $\frac{1}{b} \ln \left|a+b x^2\right|+C$.
$\text{D.}$ $\frac{1}{2 b} \ln \left|a+b x^2\right|+C$.