累次积分 $\int_0^{\frac{\pi}{4}} d \theta \int_0^{2 \cos \theta} f(\rho \cos \theta, \rho \sin \theta) \rho d \rho$ 等于
$\text{A.}$ $\int_0^1 d y \int_y^{1-\sqrt{1-y^2}} f(x, y) d x$
$\text{B.}$ $\int_0^2 d x \int_0^{\sqrt{2 x-x^2}} f(x, y) d y$
$\text{C.}$ $\int_0^2 d \rho \int_0^{\frac{\pi}{4}} f(\rho \cos \theta, \rho \sin \theta) d \theta$
$\text{D.}$ $\int_0^{\sqrt{2}} d \rho \int_0^{\frac{\pi}{4}} f(\rho \cos \theta, \rho \sin \theta) \rho d \theta+\int_{\sqrt{2}}^2 d \rho \int_0^{\arccos \frac{\rho}{2}} f(\rho \cos \theta, \rho \sin \theta) \rho d \theta$