2. There are $n$ points on a straight line, and the number of line segments on this line is denoted as $a_{n}$; there are $n$ lines in a plane, and these $n$ lines divide the plane into at most $b_{n}$ parts and at least $c_{n}$ parts. The relationship between $a_{n}$ and $b_{n}-c_{n}$ is ( ).
(A) $a_{n}>b_{n}-c_{n}$
(B) $a_{n}=b_{n}-c_{n}$
(C) $a_{0}<b_{n}-c_{n}$
(D) None of the above
2. B.

Notice that
$$
\begin{array}{l}
a_{n}=1+2+\cdots+(n-1)=\frac{n(n-1)}{2}, \\
b_{n}=1+(1+2+\cdots+n)=\frac{n^{2}+n+2}{2}, \\
c_{n}=n+1 .
\end{array}
$$

Thus, $b_{n}-c_{n}=\frac{n^{2}+n+2}{2}-(n+1)=a_{n}$.