2. Let $M=\sum_{k=1}^{n} \frac{k}{k^{4}+k^{2}+1}$,
$$
N=\frac{1}{n^{2}+n+1} \sum_{k=1}^{n} k \text {. }
$$

Then the relationship between $M$ and $N$ is ( ).
(A) $M>N$
(B) $M=N$
(C) $M<N$
(D) Cannot be determined, depends on the value of $n$
2.B.
$$
\begin{array}{l}
M=\sum_{k=1}^{n} \frac{k}{k^{4}+k^{2}+1}=\sum_{k=1}^{n} \frac{k}{\left(k^{2}+1\right)^{2}-k^{2}} \\
=\sum_{k=1}^{n} \frac{1}{2}\left(\frac{1}{k^{2}-k+1}-\frac{1}{k^{2}+k+1}\right) \\
=\frac{1}{2}\left(1-\frac{1}{n^{2}+n+1}\right)=\frac{1}{2} \cdot \frac{n^{2}+n}{n^{2}+n+1} \\
=\sum_{k=1}^{n} \frac{k}{n^{2}+n+1}=N .
\end{array}
$$