Mark against the correct answer in the following:

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

$\int_{-a}^{a} f(x) d x=?$

A. $2 \int_{0}^{a}\{f(x)+f(-x)\} d x$

B. $2 \int_{0}^{a}\{f(x)-f(-x)\} d x$

C. $\int_{0}^{\mathrm{a}}\{\mathrm{f}(\mathrm{x})+\mathrm{f}(-\mathrm{x})\} \mathrm{dx}$

D. none of these