1. Given sets
$$
\begin{array}{l}
M=\{x \mid(x-1)(x-3)(x-5)<0, x \in \mathbf{R}\}, \\
N=\{x \mid(x-2)(x-4)(x-6)<0, x \in \mathbf{R}\} .
\end{array}
$$

Then $M \cap N=(\quad)$.
(A) $(2,3)$
(B) $(3,4)$
(C) $(4,5)$
(D) $(5,6)$
-、1. B.
Notice,
$$
\begin{array}{l}
M=(-\infty, 1) \cup(3,5), \\
N=(2,4) \cup(6,+\infty) .
\end{array}
$$

Therefore, $M \cap N=(3,4)$.