Mark against the correct answer in the following:

Mark $(\sqrt{)}$ against the correct answer in the following:

$\int_{0}^{1} \sqrt{\frac{1-x}{1+x}} d x=?$

A. $\frac{\pi}{2}$

B. $\left(\frac{\pi}{2}-1\right)$

C. $\left(\frac{\pi}{2}+1\right)$

D. none of these