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
$\text{A.}$ $\int_0^1 \mathrm{~d} y \int_0^{\sqrt{y-y^2}} f(x, y) \mathrm{d} x$ ．
$\text{B.}$ $\int_0^1 \mathrm{~d} y \int_0^{\sqrt{1-y^2}} f(x, y) \mathrm{d} x$ ．
$\text{C.}$ $\int_0^1 \mathrm{~d} x \int_0^1 f(x, y) \mathrm{d} y$ ．
$\text{D.}$ $\int_0^1 \mathrm{~d} x \int_0^{\sqrt{x-x}} f(x, y) \mathrm{d} y$ ．
$\text{E.}$
$\text{F.}$