设 $f(x, y)= \begin{cases}\mathrm{e}^{x^2+y^2} \frac{\sin \sqrt{x^2+y^2}}{\sqrt{x^2+y^2}}, & x^2+y^2 \neq 0, \\ 1, & x^2+y^2=0, x^2+y^2 \leqslant t^2,\end{cases}$ 则 $\lim _{t \rightarrow 0^{+}} \frac{1}{\pi t^2} \iint_D f(x, y) \mathrm{d} x \mathrm{~d} y=$
$\text{A.}$
$\text{B.}$
$\text{C.}$
$\text{D.}$