For $n \in \mathbb { N }$, let $a _ { n }$ be defined as
$$a _ { n } = \int _ { 0 } ^ { n } \frac { 1 } { 1 + n x ^ { 2 } } d x$$
Then $\lim _ { n \rightarrow \infty } a _ { n }$\\
(A) equals 0\\
(B) equals $\frac { \pi } { 4 }$\\
(C) equals $\frac { \pi } { 2 }$\\
(D) does not exist