10. The ellipse $x^{2}+4 y^{2}=a^{2}$ (where $a$ is a constant) has a tangent line that intersects the $x$ and $y$ axes at points $A$ and $B$, respectively. Then the minimum value of $S_{\triangle A O B}$ is
(A) $a^{2}$
(B) $\frac{\sqrt{2}}{2} a^{2}$
(C) $2 a^{2}$
(D) $\frac{1}{2} a^{2}$
$\mathrm{D}$