累次积分 $\int_0^{\frac{\pi}{2}} d \theta \int_0^{\cos \theta} f(r \cos \theta, r \sin \theta) r d r$ 可以写成
A. $\int_0^1 d y \int_0^{\sqrt{y-y^2}} f(x, y) d x$.
B. $\int_0^1 d y \int_0^{\sqrt{1-y^2}} f(x, y) d x$.
C. $\int_0^1 d x \int_0^1 f(x, y) d y$.
D. $\int_0^1 d x \int_0^{\sqrt{x-x^2}} f(x, y) d y$.