题目内容
（01河南） 设 $D=\left\{(x, y) \mid x^{2}+y^{2} \leq R^{2}, y \geq 0\right\}$, 则在极坐标系下, $\iint_{D} f\left(x^{2}+y^{2}\right) d x d y$ 可表示为
$\text{A.}$ $\int_{0}^{\pi} d \theta \int_{0}^{R} f\left(r^{2}\right) d r$
$\text{B.}$ $\int_{-\frac{\pi}{2}}^{\frac{\pi}{2}} d \theta \int_{0}^{R} f\left(r^{2}\right) r d r$
$\text{C.}$ $\int_{0}^{\pi} d \theta \int_{0}^{R} f\left(r^{2}\right) r d r$
$\text{D.}$ $\int_{0}^{2 \pi} d \theta \int_{0}^{R} f\left(r^{2}\right) d r$