设 $f(x, y)$ 为连续函数，则
$$
\int_0^{\frac{\pi}{4}} \mathrm{~d} \theta \int_0^1 f(r \cos \theta, r \sin \theta) r \mathrm{~d} r \text { 等于 }
$$
A. $\int_0^{\frac{\sqrt{2}}{2}} \mathrm{~d} x \int_x^{\sqrt{1-x^2}} f(x, y) \mathrm{d} y$
B. $\int_0^{\frac{\sqrt{2}}{2}} \mathrm{~d} x \int_0^{\sqrt{1-x^2}} f(x, y) \mathrm{d} y$
C. $\int_0^{\frac{\sqrt{2}}{2}} \mathrm{~d} y \int_y^{\sqrt{1-y^2}} f(x, y) \mathrm{d} x$
D. $\int_0^{\frac{\sqrt{2}}{2}} \mathrm{~d} y \int_0^{\sqrt{1-y^2}} f(x, y) \mathrm{d} x$