2. If $a \neq b$, the simplified result of $\sqrt{2 \sqrt{a b}-a-b}$ is ( ).
(A) $\sqrt{a}-\sqrt{b}$
(B) $-\sqrt{a}-\sqrt{b}$
(C) $\sqrt{-a}+\sqrt{-b}$
(D) 0
2. C.

According to the problem, we have $a \leqslant 0, b \leqslant 0$. Then
$$
\begin{array}{l}
\sqrt{2 \sqrt{a b}-a-b}=\sqrt{(\sqrt{-a}+\sqrt{-b})^{2}} \\
=\sqrt{-a}+\sqrt{-b} .
\end{array}
$$