1. Given $a b c=1$, let
$$
\begin{aligned}
M & =\frac{1}{a+a b+1}+\frac{1}{b+b c+1}+\frac{1}{c+c a+1}, \\
N & =\frac{a}{a+a b+1}+\frac{b}{b+b c+1}+\frac{c}{c+c a+1} .
\end{aligned}
$$

Then the relationship between $M$ and $N$ is ( ).
(A) $M>N$
(B) $M=N$
(C) $M<N$
(D) Cannot be determined
$-、 1 . B$.
From $a b c=1$, we get
$$
\begin{array}{l}
\frac{1}{b+b c+1}=\frac{a}{a b+a b c+a}=\frac{a}{a+a b+1}, \\
\frac{1}{c+c a+1}=\frac{a b}{a b c+a^{2} b c+a b}=\frac{a b}{a+a b+1} .
\end{array}
$$

Therefore, $M=1$.
Similarly, $N=1$.