2. When $a<1$, the simplified result of $\sqrt{-a^{3}(1-a)}$ is ( ).
(A) $a \sqrt{a(a-1)}$
(B) $-a \sqrt{a(a-1)}$
(C) $a \sqrt{a(1-a)}$
(D) $-a \sqrt{a(1-a)}$
2. B.

From the given, we know that $1-a>0$.
Also, since $-a^{3}(1-a) \geqslant 0$, it follows that $a \leqslant 0$.
Therefore, the original expression $=\sqrt{-a \cdot a^{2}(1-a)}$
$$
=|a| \cdot \sqrt{a(a-1)}=-a \sqrt{a(a-1)} .
$$