1. Real numbers $a, b$ satisfy $ab=1$, let
$$
M=\frac{1}{1+a^{2}}+\frac{1}{1+b^{2}}, N=\frac{a^{2}}{1+a^{2}}+\frac{b^{2}}{1+b^{2}} \text {. }
$$

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