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