4.计算下列极限.
(1) $\lim_{n \rightarrow \infty }(1+2^{n}+3^{n})^{ \dfrac {1}{n}}$.
(2) $\lim_{n \rightarrow \infty } \left ( \dfrac {1}{4n^{2}+1}+ \dfrac {2}{4n^{2}+2}+ \cdots + \dfrac {n}{4n^{2}+n} \right )$.
$\text{A.}$
$\text{B.}$
$\text{C.}$
$\text{D.}$