i. Real numbers $a, b$ satisfy $a b=1$, let $M=\frac{1}{1+a}+\frac{1}{1+b}$. $N=\frac{a}{1+a}+\frac{b}{1+b}$. Then the relationship between $M, N$ is ( ).
(A) $M>N$
(B) $M=N$
(C) $M<N$
(D) Uncertain
\[
\begin{array}{l}
-1 . \text { (B). } \\
M=\frac{1}{1+a}+\frac{1}{1+b}=\frac{b}{b+a b}+\frac{a}{a+a b}=\frac{b}{b+1}+\frac{a}{a+1} \\
=N \text {. }
\end{array}
\]