10. Given
$$
\sqrt{49-x^{2}}-\sqrt{25-x^{2}}=3(x \in \mathbf{R}) \text {. }
$$

Then the value of $\sqrt{49-x^{2}}+\sqrt{25-x^{2}}$ is ( ).
(A) 8
(B) $\sqrt{33}+8$
(C) 9
(D) 12
(E) $2 \sqrt{10}+4$
10. A.
$$
\begin{array}{l}
\sqrt{49-x^{2}}+\sqrt{25-x^{2}} \\
=\frac{\left(49-x^{2}\right)-\left(25-x^{2}\right)}{\sqrt{49-x^{2}}-\sqrt{25-x^{2}}}=8 .
\end{array}
$$