设平面区域 $D=\left\{(x, y) \left\lvert\, x^2+y^2 \leqslant \frac{3}{4}\right.\right\}$, 则二重积分
$$
I=\iint_D \min \left\{x^2+y^2, \sqrt{\frac{3}{4}-x^2-y^2}\right\} \mathrm{d} x \mathrm{~d} y=
$$
A. $\frac{\pi}{24}$.
B. $\frac{5 \pi}{24}$.
C. $\frac{\pi}{8}$.
D. $\frac{\pi}{12}$.