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