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