设 $a, b$ 为常数且 $a>b>0$ ，则实积分 $\int_0^{2 \pi} \frac{d \theta}{(a+b \cos \theta)^2}$ 可转化为复积分 ．
$\text{A.}$ $\frac{4}{i} \oint_{|z|=1} \frac{z}{\left(b z^2+2 a z+b\right)^2} d z$
$\text{B.}$ $4 i \oint_{|z|=1} \frac{z}{\left(b z^2+2 a z+b\right)^2} d z$
$\text{C.}$ $\frac{1}{4 i} \oint_{|z|=1} \frac{z}{\left(b z^2+2 a z+b\right)^2} d z$
$\text{D.}$ $\frac{i}{4} \oint_{|z|=1} \frac{z}{\left(b z^2+2 a z+b\right)^2} d z$
$\text{E.}$
$\text{F.}$