5. Given $\sqrt{25-x^{2}}-\sqrt{15-x^{2}}=2$. Then the value of $\sqrt{25-x^{2}}+\sqrt{15-x^{2}}$ is ( ).
(A) 3
(B) 4
(C) 5
(D) 6
5.C.

Notice
$$
\begin{array}{l}
\left(\sqrt{25-x^{2}}-\sqrt{15-x^{2}}\right) \\
\left(\sqrt{25-x^{2}}+\sqrt{15-x^{2}}\right) \\
=\left(25-x^{2}\right)-\left(15-x^{2}\right)=10,
\end{array}
$$

Therefore, $\sqrt{25-x^{2}}+\sqrt{15-x^{2}}=\frac{10}{2}=5$.