$$\displaystyle\int_0^\pi xf(\sin x)\,dx$$ is equal to

A

$$\pi\displaystyle\int_0^\pi f(\cos x)\,dx$$

B

$$\pi\displaystyle\int_0^\pi f(\sin x)\,dx$$

C

$$\dfrac{\pi}{2}\displaystyle\int_0^{\pi/2} f(\sin x)\,dx$$

D

$$\pi\displaystyle\int_0^{\pi/2} f(\cos x)\,dx$$